I completed my PhD about a year ago and I'm currently one year into my post-doc. I had a first author in a mid-level journal and have only one citation to date. I also published a chapter in a textbook but I don't know how much weight that holds. While there are prospects of getting some data published at the end of my post-doc, it is probable that these would still be in press at the time or may not even get published at all. I'm pretty worried about the low citation index of my paper. I know I'm not ready to apply for a fellowship yet but what are my chances of nailing another post-doc.
To be frank, in molecular/cell biology your chances are rather slim. The 'standard' is almost 3-5 papers from your PhD which can be expected to be out by the end of your first postdoc plus some output from your postdoc. Of course, it is stupid to judge people only by their paper metrics, but this is what very many people in academia do. If you are lucky, you find a person who is not that quick with her judgement and searches for the hidden gold, carefully reading your letter and CV and the reference letters or is giving you a chance and invites you for interview. That can happen (happened to me once), but is rather rare.
Chances are even lower if the hiring process involves a committee panel or external reviews (as for grants or scholarships). Competition is often high, so you will usually only have a chance when you get very good/top marks from all reviewers. Because if there are three candidates who get 3 A+'s and then there is you, and even if just one of the three reviewers was only looking at publication output (chances for that will be veery high) and gave you a B or C, then you are already out and the decision will only be between the top-rated candidates.
On the other hand, a combination of a convincing letter AND the luck of being at the right time ready for the right job AND a bit of luck of hitting a person who is not overemphazising your publication output might put you into your 2nd postoc. Your chances are low but not zero.
@PuzzledBiologist said your chances are "rather slim". I think that's a little pessimistic, but I do agree that one publication at this point in your career is a moderate mark against you in that you will almost certainly be competing with candidates who have three or more first-author publications at the same point in their careers. If you're applying for a medium- to high-ranked research institution, those extra publications are considered evidence of research potential, and therefore suggest that the candidate can bring in grant funding. (I'm not saying this is fair or even reasonable, I'm saying this is how search committees tend to work.)
You say the publication is "in a mid-level journal", which is relevant because a single high-profile publication is worth several mid-level ones. "High profile" usually means one of the so-called high-impact journals, but it could also mean a publication in a mid-tier journal that has obvious impact, based on buzz or citations or whatever. It doesn't seem that helps here, though, since the publication has "only one citation to date".
That said, having one publication is a lot better than having none, and you could be a strong candidate in enough other ways to overcome that. Academic careers that emphasize teaching also tend to de-emphasize publication counts, so that also may help focus your search. Your research could be a particularly good fit for a particular department based on their needs or strengths or weaknesses. A strong and personalized recommendation from a mentor, that specifically addresses the lack of publications and explains why it doesn't reflect on research ability, can help enormously.
(For example, I know of a candidate with a relatively modest publication count, whose interests fit departmental needs; a member of the search committee phoned a friend who knew the candidate, who said something like "Oh, they don't have a lot of publications because they're so busy helping everyone else. They're the unofficial troubleshooter of the whole department, they solve everyone else's problems first.")
The strongest candidate for a research university position will have multiple high-profile papers plus strong recommendations, a good departmental match, and a personality that fits with the hiring group. The fewer boxes you can check in the list, the harder the chances become, but there are definitely boxes other than publication count.
Edit to address two other points in the question:
it is probable that these would still be in press at the time or may not even get published at all.
"In press" is perfectly fine. In-press manuscripts count exactly as much as published manuscripts do as far as search committees are concerned -- if not more, since they are evidence of active, ongoing work.
"In preparation" is better than nothing, since it suggests that you consider your work complete and that it has a chance of being published and impressing granting committees, but it's not great.
what are my chances of nailing another post-doc.
I think odds of getting a second post-doc are pretty high, just based on the little we know here. You'd need a decent reference, but you'd be primarily competing against new PhDs and the expectations for publications are much lower (since post-docs don't have as much need to get their own grants, their portfolios don't need to be as strong coming in).