I put a Vivitar tele converter on my 70-300 lens. It went on easy but wont come off. How do I get the thing to come off my lens? I have a Nikon D3000 camera.
If you have already given it a good twist it is likely the lens mount is jammed at the locking stud. Twist it firmly in the "on" direction, then press the release button and try to remove it again.
It will come off easily. If it doesn't, there's a reason. Applying any excess force will likely just break something, and "something" in the same sentence as "Nikon" usually equals "expensive"
The teleconverter locks onto the lens just the camera body does and it features a switch that you can push in order to release the lens again by twisting. I don't know what exact model of a teleconverter, but this video features a Vivitar teleconverter for Nikon. The link will start the video at the 1.33 mark with the teleconverter's lens release nob clearly visible at the left hand side of the frame.
If you already know about the switch but can't press it down for some reason (as Dan suggested could be the case in the comment) please send a picture so that we at least have a chance to assess the situation.