Try putting the throttle in the stop position and then try and click the ball in. You should hear a snap sound which will show you it's clicked in properly. You may need to move the throttle into the run position ever so slightly and then it should click in easily.
If you can upload a photo we can check if something else might be wrong.
Remember this process is much easier when the carby is apart with just the cam sitting in the carby body. Also remember the ball spot on the cam needs to be in line to where the throttle cable snaps into it's guide.
