You can't assign ringtones to individual contacts for SMS. There is a work around that involves setting up profiles for each contact but it is not 100% effective and I am ready to dump it myself. In the menu of each contact is a link to set a custom ringtone for that caller. You can also go into the active profile and set a single ringtone for ALL SMS.
Here's the work around if you wanna try it. How to set custom text message tones to contacts