Usually the soundserver is just running. (A terminal window will be on screen showing " Waiting for connections ... " on its last line.)
If it isn't, then open a terminal window (by clicking Terminal in the dock) and use the <up arrow> to find the following command (or, if it has not been recently issued, just type it and hit <return>) ...
In dire circumstances, the Mac G5 may need to be rebooted and this can take a long time. Only reboot if necessary. Unplug the firewire-connected Motu before rebooting. The Mac will automatically reboot logging into the 'CAVE' user. (Prof. Cayley has the pw for this account. The CAVE user is not an administrator for the G5. Prof. Cayley also has the Administrator pw.) Remember to plug the Motu back in after reboot is complete and wait for the Motu to initialize.
Windows PC that runs grexecd.exe
The Windows PC to the right of the Cave must also be running a program called 'grexecd.exe'. Again, usually this is just running with a terminal window showing various successful network connections.
If there is no such window open, you may need to double-click the icon with the same 'grexecd.exe' name, in the top right corner of the desktop.
If this computer has to be rebooted, you should login as 'cavedemo' with the pw 'cavedemo'. Usually, after such a login, grexecd.exe will be run automatically.