Linux or Mac?
Do you have more than one fpc version? Or did have an older once?
During compilation, fpc will only look for the ppu (at least with regards to rtl). The ppu must be created by the same compiler version, as the one that is attempting to use it. (including same revision (same ppu version), if it is a svn build)
try compiling with -va (large amount of output). ANd see which files are searched, and why they are rejected. (also which fpc.cfg is used)