Sending SMS text messages using iMessage

I am trying to send SMS text messages from my iMac to non-Apple smartphones. For example, I have a client with an Android phone. I want to send him a text message from my deskstop computer (an iMac 27) to remind him of an appointment. I can't do that because he is not a registered user with iMessage. Before the advent of iMessage, I could send SMS messages throught iChat. Now I can't. What can I do to regain this capability?

Can you read through this and see if it helps. I will try it out tonight on my iMac..