Click on Message Center in the sidebar
In the top right hand corner you will see your current SMS balance
Next to the balance is a button where you can buy more messages
Follow the steps on the new page that opens to complete your purchase

Click on Message Center in the sidebar
In the top right hand corner you will see your current SMS balance
Next to the balance is a button where you can buy more messages
Follow the steps on the new page that opens to complete your purchase