For both of the two games which currently support MacOS X, Geneforge and Avernum 3, they will currently work for only the user who installed them, not for any other user sharing the same system. This is due to a bug in the Unix file permission of the installed version.
The obvious workaround is to install multiple copies. The fix, is to go in and change the permissions so that the proper files are world writable.
Here is how to do it from the command line. (This assumes that you are the person who installed Geneforge or an administrator.) You can probably achieve the same effect using the Get Info box.
Geneforge:
#cd to the directory where you installed #Geneforge, e.g.,
cd /Applications/Geneforge\ v1.1\ \306\222/
#Then into the Geneforge Files directory
cd Geneforge\ Files/
#Add world write permission to all of the Save #directories:
chmod 777 Save*
and their contents
chmod 666 Save*/data
chmod 666 Save*/header
chmod 666 Save*/items
chmod 666 Save*/jounal
chmod 777 Save*/temp
Note that you may need to periodically rerun this script as Geneforge seems to reset the permissions on the journal files.
Note also that the multiple users will save the same 16 save slots; you will need to work that out for yourself. However, be aware that the Autosave will get written over for each user.
Avernum 3:
#Goto the Avernum 3 directory
cd /Applications/Avernum\ 3\ v1.1.1\ \306\222/
#Then into the Avernum 3 Files directory
cd Avernum\ 3\ Files/
#Add write permission to all files:
chmod 777 *
This should do it.
Note: you cannot back up registration for Avernum 3 using the terminal cp command. This does not properly copy the resource fork.