Open a "terminal" session to get a command line.
csh should be in /bin/csh assuming you have it installed. It came in with my Debian and RedHat/Fedora installations. If you don't have it you can get it by installing the tcsh package.
If you're familiar with command line in BSD/UNIX you shouldn't have much trouble with it in Linux. The command names are genearlly the same although some have different flags so keep the "man <command>" in mind if you get stuck on usage.
The default shell in Linux is bash (Bourne Again SHell) which is similar to ksh and the posix shell.
If your C script doesn't have an interpreter line at the beginning:
That may be your problem. (# sign is included - this is special syntax). Also may be an issue if your permissions don't include execute (e.g. if read/write rather than read/write/execute). I don't really use the GUI much so don't know either of those IS the issue but they would be at command line so it is reasonable to suppose they would be when double clicked.
P.S. ugh! c-shell