Is it the lever attached to the seat that allows the seat to slide back and front? If it is it may be rusted and not function. Or is it the differential lock lever (pedal) close to the floor board? Or something else. Can you post a picture.
You just step on it when the tires are spinning and it will lock them so that you get equal power to both wheels. Be sure you get off of it when you are moving again and don't try to turn while they are locked. It's not good for the rear end!
Usually when I engage the differential lock I am in very sloppy conditions. I have no problem turning either my 2120 or TN75. On the TN, the differential lock , locks both front and rear axles.
You MUST not have one rear tire spinning faster than the other when you step on your diff lock pedal. If you do there is a very good chance you will shear off the pins. This style of diff lock is a jaw type, not a disc type. It can be engaged when moving as long as you are traveling in a straight line with no wheel slippage. One wheel spinning faster than the other + stepping on your differential lock = broken parts.