Before installing a lens, make sure there are no obstructions inside the projector (like the original plastic lens mount protector), and remove any back cover or rear cap on the lens itself.

Look at the metal mounting flange inside the projector, and at the tabs on the lens itself. There should be three tabs/slots visible.

Two tabs/slots are about the same size -- they'll be on top when the lens slides in.

One slot/tab will be about 2/3 the length of the others. That one goes "down" as the lens is inserted.

Insert it such that the smallest tab of the lens mount "foot" faces down, make sure the lens is completely seated "in-plane', then gently rotate clockwise about 45 degrees -- you should hear and feel a definite tactile click when it locks into place.

Don't force it! If it isn't rotating easily, something must be in the way.