I installed pgsql v9.1.8, used Stackbuilder to download pgAgent v3.3 and tried to run the installer. After getting to the window where the details for the server are confirmed and you enter the postgres user password, you click next - then I got this error (note: this is taken from the crash report):
Date/Time: 2013-04-02 22:29:26.686 +1100 OS Version: Mac OS X Server 10.5.8 (9L34) Report Version: 6 Anonymous UUID: 7D8CEC23-9DF5-4131-8D3F-B534C6C84AFC