Expanding on Jon's excellent explanation of where things are, here's how they work:
Had your plug-ins been located in ~/Library you'd have been able to print when you logged in to another account, because they'd have affected your log-in account only, whereas the fact that you could not print from your other account told us that whatever was causing your problem was something that affected all accounts...perhaps something in /Library (where you ultimately found your plug-ins).
Happy to hear that you've gotten things sorted out.
