I know that many of the Cinema4D users had or are still having the problems I had.
The problem I am talking about is the problem when Cinema4D won't recognize the Sketchfab C4D Exporter Plugin.
How did I solve the problem?
Well, you will be surprised, but I could solve the problem by having a newer version of Cinema4D. I had Cinema4D R13 before and haf that problem. I and the support team were thinking what could cause the problem until I became it solved by having the version "Cinema4D R16". So far as I saw in the forums, the plugin seems to work at R16 and up only.
Hope I could help you all with this post and have fun modelling and animating!