First, look at the PTO on the tractor. It should have a groove around it's circumference about 2/3rds in from the end of the shaft.
Second..make sure the tractor key is in your pocket!. We don't want any accidents, do we?
Then, look at the implement PTO shaft (U-Joint end that goes on the tractor). There will be a pin (look for a spring perpendicular to the inside splines on the U-Joint, or there will be a rotatable ring close to the the end of the U-Joint nearest the PTO shaft.
Once you've found the locking mechanism, you're half way there.
On the tractor end of the PTO shaft, you must push the pin down or rotate the ring to release tension on the detents (same principle as the ball detents on a rachet wrench). Once you release the tension, the female splines on the U-Joint should go on the tractor PTO shaft fairly easy.
Make sure you hold the release mechanism (pin or ring) until the splines are fully engaged. Once you've got the PTO shaft completely on the (tractor) PTO shaft, give it a good yank back to ensure the detent is locked in the groove on the tractor's PTO.
If the tractor's PTO shaft and implement shaft's splines are not lined up, you should be able to rotate the tractor shaft to align them.
If it doesn't slip together, look for burrs on the splines.
Now, I'm assuming the tractor and cutter were rigged up at one time and that Benya has a standard PTO shaft size and spline. If it's a grey market tractor, it may not have a standard (1.375") shaft size or spline configuration.