I think more information is required here. I also run C++ 6.0 with SDK 0.9 for my Series 60 development (the free download from forum Nokia) and have resource files for menus, strings etc without any problems. Has this started since you added something extra into the resource file?
I found out what the problem was. It was so simple I feel a bit stupid. I was editing the wrong rss -file (one located in the group-directory) not the one the mmp file was pointing to (data-directory). I don't know why I had two versions in the different locations - probably my mistake also.
So you really have to read carefully what is there in the mmp file.
After I found out my mistake I got another error "Unresolved external" when reading a resource id but after adding some libraries in the mmp-file I was able to fix that thing also so I am currently happy with my Symbian application.