Make sure you are using Eclipse 3.8.2 or 4.2.2 (or better). Previous versions are known to have issues with the ADT.

Select File | Import | Install | Install Software Items From File and choose a locally downloaded copy of 
https://github.com/cgeo/cgeo/tree/master/main/project/eclipse%20installation/cgeo%20eclipse%20components.p2f.

This way you can easily install all necessary plugins for c:geo development.