Click on your name in the upper right and click the drop down.  Select My Profile from the menu

Enter the number of the mobile phone you wish to recieve the text messages on.

Click the Opt-In slider next to the mobile number field.

A confirmation message with a verivication code (OTP) will be sent via text to the mobile phone you listed.  To complete the Opt-In process, enter the code into the box and click Submit.  If you do not recieve a code, you can click "Resend OTP"