My Seirawan chess program is here:

http://hem.passagen.se/melki9/seirawanchess.htm

Mats

Note that there's been an update to my Seirawan Chess program. Earlier it too often weakened the wings by moving the a- or h pawn two steps. I also added an alternative variant where  the external pieces are introduced automatically at the first and second piece moves. This is a simplified form of S-chess.

http://hem.passagen.se/melki9/seirawanchess.htm

/Mats

Note that the XBoard interface also supports Seirawan Chess, and that one of the engines that is included with it (Fairy-Max) actually can play it. XBoard installs for Mac are available from WinBoard forum ( http://www.open-aurec.com/wbforum/viewtopic.php?f=19&t=50428 ).