T Mount problem ?
The Opekas have a set screw in the side of the T-mount adaptor, loosen the screw and it should come apart. However, if they sold it to you with a T-mount to Nikon adaptor it should fit you camera.
To use the 2X extender, remove the T-mount adaptor from the lens screw the 2X extender onto the bottom of the lens, then mount the T-mount adaptor to the 2X extender.
The lenses are pretty poor quality, you can probably do better with a good 200mm lens and just enlarge it to the same object size.