When you put on the lens you have to check to see if the locking tab, locks into the lens locking tab as shown below.

Now to release the lens make sure to pull the release knob straight up making sure the release knob does not tilt when pulling up on the knob as shown.

Not like this

Pull (Red) release knob down and hold. Then hold (green) plastic area, And twist  lens to the right to remove lens from adapter. (Yellow) Is the portion of the lens that will open and close the aperture of the lens.