Cant add Terrain Toolkit

It has always worked before but for some reason now it is not. When I drag the c# scrip onto my terrain now it keeps saying:

Cant ad script Cant add component terrain toolkit because it doesn't exist. Check to see if the file name and class name match.

I deleted it from my project and re-imported it again but the same thing. Not sure what to do. Love using this as it makes my life so much easier when importing custom built heightmaps.