What do you mean? The PL/SQL Developer (PLD) does work correctly on a 64 bit Windows (I'm using it on Windows 7 64-bit since PLD version 5 or at least 7) as any other 32-bit application. The only requirement (as said in the thread I pointed to) is to have the 32-bit Oracle Client correctly installed (or placed/pointed to, in case of an Instant Client).
Edit: I wrote "does work correctly", where I should have written that it does work just as on 32-bit Windows, which means same issues/bugs.