Sending a SMS to a Messenger contact from your computer:
(1.1) Using Yahoo! Messenger on your PC, right-click your contact and select Send SMS Message (also available on Actions menu). For Mac users, press the Control key, click on your Messenger contact and select Send SMS.
(1.2) Enter your contact’s mobile number and click OK. For Mac users, input the number and click SMS.
Note: Make sure you add the country code for international numbers.
(1.3) Type your SMS message and click Send. Mac users hit the Return key. Your contact will receive your message on their mobile phone.
Yahoo! Messenger for SMS is currently available in the following countries: See countries