Stephanie's idea of using another dog to show the way is excellent. However, if your dog doesn't get the idea, then you need to try a compound approach. Your dog is afraid. Therefore, link your dog to the another that she likes using a "daisy chain" which is a coupler with snaps on both ends and have yours follow the other one around for a period of time. After your dog gains some confidence, then try the stairs in this mode.
Google: Brandon McMillan and daisy chain and watch some videos how he approaches a shy/timid/fearful dog by letting a more adjusted dog pave the way.
Also, I don't know how you are using the treats. But try laying the treat on the stair itself, starting at the bottom of the stairs first. Going up is less scary than down. Lay it initially just within the dog's reach, then move it further away. Make sure they are high-value treats. Take it slow and continue to reward and praise. Then do the same with going down.
Good luck!