If you're wanting to send an SMS, however the list you want to send to isn't showing up as an option in the send process, it is most likely that the mobile phone field in that list is not set up correctly.
Check your mobile phone field is set up correctly
- Click lists and forms on the left hand side
- Find the list you want to send the SMS to and follow it over to the right side and click the 'view contacts' button
- Again on the right hand side, click the 'spreadsheet' button (it looks like a pencil)
- In this spread sheet view, find the mobile phone field and hover your mouse over the column header. Click the small white icon that appears and choose 'edit'
- In this dialogue, click the drop down box opposite the 'content type' and make sure it is set to 'mobile phone number'
- Click the green save button
Your list should now show up when you go to send your SMS!