Not that it's integrated with Mambo, but I highly suggest
Kayako eSupport. We are currently beta testing v3 (rc1 will be our shortly) and it is simply amazing. Right now our help desk is powered by v2.
From the other thread it appears as though you are using ModernBill. There is a login module for Kayako v3 so that when logging in, it actually checks info against the ModernBill user info so users don't need to register twice.