Truepianos ALWAYS has only 1 .dll called TruePianos.dll. The modules are a different file type and are installed elsewhere....so even if you have one module or all of them, there will only be 1 .dll.
I suspect (just guessing) you have the Sonar Truepianos installed (the one that came with Producer) and then you installed the "full version" also installed except the installer put the TruePianos.dll for the full version somewhere else on your system.
If it works in standalone, it's on your system. Sonar is just looking at the "wrong one." Did you uninstall the Sonar one using the Add/Remove Programs?
Perhaps do a search for TruePianos.dll on your system and see what results you get. It would be helpful to know if you have multiple copies on your system.
As a side note, AFAIK, there is manual for TruePianos in your account on their site. It's a separate .pdf download. It might not be helpful, but there is documentation.