of course, you'd need to be running Terminal.app
Another way - in a file viewer window you'll find Applications as a top level item, inside that is Utilities, inside that are a bunch of programs. One is called Terminal, and another should be called X11 (I think - don't have a mac handy just now).
Your install is likely fine - however X11 is an optional package install on the CD's (if using panther). If Tiger, install is from on DVD (hooray!).