A bit old thread here, but I had the same problem in CS3 and couldn’t find the solution anywhere. However, I finally found out one solution for this that worked for me at least, so I’ll reply here so it is visible for others too.
Start creating the button from a clean Photoshop file. Do not continue it from some menu that is opened from Encore. Right now I have two identical Photoshop files that have the layer names as they should for buttons (with (+) and so on). Other one, which is continued from Encore’s menu, imports as a menu. Another one is a clean new document in Photoshop, but the layers are drag & dropped from the previous file. This one imports as a button.
There must be some extra info somewhere in the file that makes the difference, but couldn’t find it yet.