The documentation for more recent releases is now found on our "docops" site.
InterTest and SymDump, both Batch and CICS, are found in an integrated piece supporting all 4 products here:
This includes a section for messages and codes for InterTest CICS, InterTest Batch, SymDump CICS, and SymDump Batch.
For FileMaster Plus for MVS, the docops link is:
This includes install manuals, but historically FileMaster Plus has not had a message manual. The explanation is that the errors are "self-documenting" by the error messages themselves or the Help facility. This I would suppose would be true most of the time, and if not this is where I would come in if you open a case.