You can plug the CS50 into the headset port on the phone.  Make sure the black cable that came with the unit is coming out of the bottom of the charging base where you see the full telephone picture.  Plug the other end into the headset port on your phone.  On the left side of the charging base you will see a plastic screw head.  Put the number 4 next to the notches on the side putting the number 4 where a 3 would be on a clock.  Then when you take the headset out of the base, hit the silver button on the headset, then the headset button on your phone and you should have dial tone in the headset.  If not, put the headset back into the base, hold in the 2 arrow keys located on the back of the base at the same time until you see the red light on the front of the base flash, then release.

Hold down just the black volume/mute button on top of the headset for 10 seconds, then release.  Then unplug the AC power from the base (not the wall) for 10 seconds, then plug back in and then try your headset.

There is also a hook switch cable that you can get for that particular phone model that will alleviate the need for a lifter.  The part number is APC-4, the firmware in your phone must be 8.3 or higher and if you use this hookswitch cable, you would need to move the dial on the left side of the base to 1 instead of 4.

Attached a copy of the Plantronics CS50 manual as there are a bunch of weird settings switches and dials.