Hello:

Many of these are not scripted, so it is really just to make it as clear as possible. You can leave the degree as M.S.Ed. or M.S. Special Education or M.S. Special Ed. Then for the certificate, I recommend Certified in Assistive Technology or CERT Assistive Technology or CERT Assistive Tech. I don't think you can get away with abbreviating Assistive for most audiences, but if his is all within an industry where people would know what AT is, hen you could use that. But for most audiences, I don't recommend it.

I hope that helps!