I fiddled around with Winbuilder on a server and got "Error with mounting source! (Install.wim)" message everytime, having checked my imagex versions and tried different sets of souce DVDs.
Finally i figured out my problem:
NEVER, ever, use a virtual drive (if you're to lazy to write the whole path like me) with SUBST command, as imagex will fail (you know, like symlinks in linux, the subst command in windows).
Don't use network shares too, as imagex will fail.
Best solution is to try imagex on command prompt like:
d:\>MD testdirectory d:\>imagex /mount d:\path\to\your\source\dvd\files\install.wim 5 d:\testdirectory
and see what it does.
Hope this will save some headaches to some people.