You'll also need to install a version of Tcl/Tk, just like UNIX users do.
If you don't have a Developer toolkit then you can download a precompiled binary version of the Xfree86 system.
A detailed description of how to do this is given here:
When building a local version of the Xfree86 system the approach goes roughly like this:
mkdir xfree86 cd xfree86 cvs -d :pserver:anoncvs@anoncvs.xfree86.org:/cvs login (use the password 'anoncvs') cvs -d :pserver:anoncvs@anoncvs.xfree86.org:/cvs checkout xc cd xc make World sudo make install (use your Mac OS X administrator password)
fink install tcltk