The SOS Client is not a native Mac app, so it needs a running X11 server.

X11 should be on the installation discs or you might find X11 from Apple's developer site. You should be able to find it with an X-code bundle. Also, there were some versions of OS X which had problems with X11 and it was fixed in a branch of this code named XQuartz - http://xquartz.macosforge.org/trac/wiki

In any case, once X11 is installed / configured, you must start the X11 server. After this you can either start an xterm and run the sos binary from a command line (/Application/SourceOffSite/sos), or just double the /Applications/SourceOffSite/sos icon)

Some other helpful hints :

a) viewtopic.php?f=10&t=5599&p=22455 -

Opening SOS with X11

1. Go to your Utilities folder (or where you installed SourceOffSite)
2. Open the SourceOffSite folder
3. CTRL-Click or Right-mouse click the sos file
4. Select Open With > Other...
5. Find X11 (in your Utilities folder), don't forget to click the checkbox Always Open With
6. Double-Click the sos file in the SourceOffSite Directory

b) configuring diff/merge

viewtopic.php?f=10&t=2782&p=10228
viewtopic.php?f=10&t=10130&p=41230

Also -- be sure you have the Mac client for Intel Mac, if you have Intel Mac.