Hi all I have a pentax K-x. I have just bought a secondhand Marexar auto tele converter 2x for pentax-k

I have set the camera up so that i can take photos with it, but just one problem, the converter locks in to the body ok, but when i then put the lens on to the converter it is not locking. on the outside of the converter is a small metal pin that i belive locks the lens to the converter but it dont seem to lock. any ideas please, i am new to all this, anyones help would be great thanks