I checked the User Manual for the device and page 65 and 66 have steps for editing a contact's information. I've posted the steps below.
- Open your contacts
- Press and hold on a contact to bring up a menu
- Tap on Edit
Inside here there should be a section called Ringtone that you can tap on to select a ringtone for that contact. I don't have the Samsung Dart with me right now so I can't test this resolution.
Please let me know if this works!
This could be affected by where you saved your contacts to. When you edit the contact, does it say if it is a Google, Phone, or T-Mobile number? If not, you should be able to customize your contact list to see where the contacts are. With the contacts program open, press the Menu button. There should be an option for View either right there or if you select on More. I think only certain accounts will have that option.
Let me know if this helps!