There are two "latest" versions of Cura. You have the 15.04 and the 15.10 version (renamed to 1.99). The 15.04 version uses a very limited plugin system which only allows you to perform post processing on g-code.
The 'new' Cura on the other hand has a very advanced plugin system that allows you to add new tools, do post processing, add new rendering types, new file types and even change the backend.
Both work quite differently with regards to how to add plugins.
That being said; What did you try? (and didn't work)