Linux

On modern Linux systems, it is usually easy to find.  An icon should look
like a computer monitor, or perhaps a paneless window (i.e. just a frame, no
cross pieces).

Some places to look might be:
   Applications
   Applications -> System Tools


OS X

On a mac, the useful programs can generally be found (using Finder) at one
of the following locations (listed in the order of initial usefulness):

   Applications -> Utilities -> XQuartz
   Applications -> Utilities -> X11
   Applications -> Utilities -> Terminal

The XQuartz and X11 icons should have a big 'X' in them, while the Terminal
application's icon looks like a simple monitor or paneless window (i.e. just a
frame, no cross pieces).

It is usually convenient to drag the desired program to the dock, since
(hopefully) it will be used more often than iTunes, say...

Comments:

   AFNI programs run on the X11 window management system, which is the default
   on most Unix systems.  But while macs can use the X11 system, it is not used
   by default.

   XQuartz is basically just a newer version of X11.

   Some people prefer using the Terminal application rather than the xterm that
   starts up when using X11 or XQuartz.  That is up to you.

10.6 only:

   Note that on 10.6 the Terminal application will not automatically start X11,
   meaning starting afni might not do anything.

10.7 and 10.8:

   Terminal starts X11 automatically on 10.7 and 10.8 (but not on 10.6).

10.8 only:

   On a new 10.8 system, the first time one of these programs is opened
   (or possibly after the program is opened and then the user runs 'afni',
   for example), the user will be prompted to install XQuartz.  A web page
   will be opened where one can download and install the .dmg package.

   XQuartz (and X11) do not come by default, so there is a brief extra
   step to install it.