Usually, "servers" are thought of as systems in some other room that you access as a "client" from the machine in front of you; for X11, the machine in front of you is the server, and remote machines are clients. Because of this unusual situation, it's easy to get confused, either as the author of a question or as a reader.
Laura, when you say you're "trying to connect to the desktop of my sgi origin ... using x11", do you mean that you want to run a program on your Mac and have the window appear on the SGI origin's display? Or that you want to remotely log in to the SGI origin, and have X commands executing on the SGI origin display on your Mac?
If you want to remotely log in to the SGI origin and have applications started from there display on your Mac, the easiest thing is to start a Term (xterm) from the X11 "Applications" menu on your Mac, then use "ssh -X <yourSGIsName>" within the xterm window to log on to the SGI origin. SSH will ensure that the DISPLAY value on the SGI is set to use your Mac's X11 server. If you can't log in to the SGI origin using "ssh", post back...
If you want to use the SGI origin's display to show windows for applications running on your Mac, and you have Remote Login enabled on your Mac, just reverse the process -- from an xterm running on the SGI origin, run "ssh -X <yourMacsNameOrIPaddress>". SSH will ensure that the DISPLAY value on your Mac is set to use the SGI's X11 server. If the SGI doesn't have the "ssh" client program, post back...