Message sent from iPhone but Mac unable
I have a MACBook Pro and and iPhone 4s both with the Message App. The other day I was given a phone number and tried to send a message via my MAC using the Message App but it said it was unable to send the message. Yet I successfully sent the message to this person via my iPhone's Message App and he responded. How can I send him messages with my MAC's Message App.
MacBook Pro, OS X Mountain Lion (10.8.2)