Desktop computers: Troubleshooting wireless keyboard and mouse issues in Mac OS X v10.5 (Apple KB)
Some desktop computers with wireless keyboards or mice are shipped with the devices pre-paired and the batteries pre-installed. As long as the wireless devices are powered on when the computer starts up, no additional steps should be necessary to make them work. If one or both of the wireless devices do not function as expected, use the information below to identify and resolve the issue.
Note: These instructions work with Mac OS X v10.5 Leopard. For troubleshooting wireless keyboard and mouse issues in Mac OS X v10.6 Snow Leopard, see Desktop computers: Troubleshooting wireless mouse and keyboard issues.
Read more: http://support.apple.com/kb/TS2662