There isn't a U3 forum, so I think that because it is like a portable version of the main avast 4 home/pro version you could post there and ensure you put U3 problem (or something like that) in the subject. You would certainly need to be more detailed in outlining the problem as this really doesn't help identify the problem.
What I want is to use PortableApps in combination with Avast on my U3 stick. Mainly because the U3 software just sucks and crashes.
But I cannot get it to work :/
I can't recall there being a non-U3 version of avast and I have been on the forums for over three years.
The only other version vaguely related to portability would be the PDA version but that wouldn't work on a U3 drive.