To send a text message:

1. Click the down-arrow on the side of the New button.
2. Click Text Message (SMS).
3. This opens a box at the bottom of the page for you to write your message. Please note: that this box can contain a maximum of 152 characters (including spaces).
4. When you're done, press Enter on your keyboard or click Send.

From your Yahoo! Address Book

Click the mobile phone icon next to a phone number in your address book. From there, just enter your message and click Send.