In Berne Convention countries (which comprise almost all of the world), the source code is copyrighted as soon as it's written. Without a license, you can't make a copy (or do other things that can vary by country). So, without a license, you can't legally use it. You might be able to argue Fair Use in the US, but don't count on it, and check with a lawyer first.
Using it as a basis for your own development is (at least in the US) creating a derived work, and that's still under the initial copyright, so you still can't use it. You can start with the algorithms (which should be explained in the text part of the paper) and write your own.
The only way you can legally touch it is with permission. If you can't find a license, you can't use it. You can write to the copyright holders and ask for permission. You can suggest that they put some sort of open source license on it, but they aren't required to.
Using it internally and not revealing what you're doing will normally keep you from being sued, but it's still unlawful. Publicly using it would be an openly unlawful act, and I wouldn't try it without consulting a lawyer.