A postdoctoral associate/fellow/whatever is usually an employee. While the purpose of the postdoc is educational, it is not "education". A postdoc is (usually) not taking classes, following a curriculum, and completing other activities toward the earning of an academic degree/certificate/whatever. Postdocs should have a contract, do not pay tuition (or have tuition paid for them), have a definite end date to their employment (although that can be renewed), etc. Since a postdoc is an employee, it should go under employment.
However, in some fields and countries, it may be custom to place the postdoc under education because the postdoc is the journeyman stage of your academic career. Ethically, it should not be listed in both places.