My keyboard and mouse won't sync to my iMac, I've changed the batteries, any ideas?

Do you have issues with any other Bluetooth devices? Reseting the SMC usually resolves Bluetooth issues.

How to Reset SMC: http://support.apple.com/kb/ht3964

Resetting the SMC for Mac Pro, Intel-based iMac, Intel-based Mac mini, or Intel-based Xserve

1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.

Note: For Intel-based Xserve computers that are not responding, you can shut down locally, or by using remote commands, or by pressing and holding the power button for five seconds.

My mouse connects just fine, so Bluetooth is working. Just can't connect to keyboard after changing the batteries. I don't own a USB keyboard so I am dead in the water!