XULRunner operates a bit differently under OSX.
The Mozilla Developer Center contains information that you'll need. There seems to be issues with the community provided package (currently 18.104.22.168), although ver 22.214.171.124 has been successfully used.
The staff client data from the ./staff_client/build directory must be placed somewhere on the machine (e.g. ~/Desktop/Evergreen_Staff_Client).
XULRunner must be called via the full path to the binary, followed by the install command and the path to the client data.
/Library/Frameworks/XUL.framework/xulrunner-bin --install-app ~/Desktop/Evergreen_Staff_Client
This command should exit quietly.
A folder named OpenILS under /Applications, with a launcher named open_ils_staff_client, will be created.
If you already have ver. 126.96.36.199 installed, per the 188.8.131.52 release notes, you'll need to remove the entire /Library/Frameworks/XUL.framework directory before reinstalling. In addition, you may also need to remove the previous xulrunner-ver-mak.pkg file from /Library/Receipts/