first, i would suggest setting up fink if you haven't already.  you can go
to http://fink.sourceforge.net/ and follow the instructions for
downloading and configuring fink.  fink will make it a lot easier to
update gmt, etc... as new versions become available

second, using fink, make sure you have working copies of all the required
software (i.e. gmt, perl, tcl/tk, gawk, ghostview, imagemagick).  use
commands like
        fink selfupdate-cvs
        fink install gmt gv imagemagick

you've probably already got an xwindows system up and running, but if not,
you'll need to.  i recommend the X11 beta that is free at the mac os x web
site.  the fink web site has instructions about how to make fink and
apple's X11 work well together.

OK, now you are ready for igmt.  i found the directions on the
manual_v1.2.pdf file pages 7-8 superb (this file is available at the igmt
website).  in particular, follow the instructions for telling igmt to use
the bash shell (which is included with os x at /bin/bash) instead of the
korn shell using perl.

then set an an environment variable to point to the igmt directory
(wherever you decide to put it).

next, the igmt script will want to call 'wish' from the tcl/tk toolkit.
type which wish to figure out where it is on your system.  since i use
fink, wish for me is located at /sw/bin/wish which is not the default
setting.

do the same kind of thing for the gmt binaries.  find out where they are
and tell igmt where to find them.

most of what i just said was a poorly explained version of the manual.  if
you've got a specific question about something, let me know and i might be
able to help.