I can only speak from experience on window OS's, but it sounds like either:
1) fbc was never installed
2) you're not in the right directory to use fbc
3) you never added fbc to the system environment variable PATH
More than likely your issue is coming from 1 or 3. If you don't know how to do 3, don't attempt it.
A work around for you would be to call the fbc executable with an absolute path:
C:\Program Files\QBasic\bin\fbc -lang qb -dll "c:\documents and settings\DLL_text_QB.bas" |
Obviously that path is more than likely incorrect, but since I don't have QBasic, I have no idea what the legal path is, nor do I know exactly where you installed it.
Edit: If fbc hasn't been installed into the system, you need to install it before you can use it (duh).