Hi Carm,
At least you had them in the first place! Installed the 737-600/700 this afternoon, having the same issue, despite having (attempted to) install FSUIPC as prompted (repeatedly) by the game.
Fix I've just found in a different part of the forums says that uninstalling the 737, then every single component of FSUIPC, before re-installing the 737 and the latest version of FSUIPC (available HERE) seems to work - trying it right now, will let you know...
Otherwise, anyone else, solutions would be awesome!
TovarichZulu
POST-SCRIPT: the link I posted.... is that just a single DLL file? I may be being incredibly dense here. Do I have to buy the FSUIPC listed on the sim market link on the above site?