Update to the latest version of Mountain Lion. Update to iOS 6. Then in the Messages application on your Mac, go into Preferences. You should see a list of e-mail addresses and phone numbers you can tie together.
Make sure you have iOS6 on the phone and 10.8.2 on the Mac... then go to the phone and turn iMessage off then back on. That should cause a popup on the Mac that allows you to add the phone numbers to the Mac for Messages use.
Make sure you have iOS6 on the phone and 10.8.2 on the Mac... then go to the phone and turn iMessage off then back on. That should cause a popup on the Mac that allows you to add the phone numbers to the Mac for Messages use.