First check you can join an existing online flight. If you can't, something is missing from the Win 7 fix so make sure you've followed all the instructions. If it helps, there's a set of screenshots taking you through the process - they're on page 5 of the Windows 7 install fix thread.
If you have any add-on aircraft in your installation that will cause aircraft mismatches - most games are stock aircraft only.
If you're still having problems you need to ask the online flyers. Hope this helps!