The 790 has a knob under the seat that adjusts the rate of drop for the 3 point. It may be turned to where the lift is locked. I use this feature when sharpening my finishing mower blades. I put saw horses under the mower and then turn that knob to keep 3 point from leaking down.
Should be a lever on the right side of the operators seat that goes up and down with numbers along the slot the lever moves in. I take it you have tried moving this lever and the rockshaft has not responded?
Have you tried applying some strong downward pressure on the lift arms with the rockshaft control lever in the full down position? It may just be a matter of the lift arms being stuck in the up position due to a long period of none use and the fact that the rock shaft links and arms are not installed which normally place enough downward pressure to pull the hyd. lift arms down.
Keep in mind that the rockshaft hyd. lift arms are a ONE WAY servo. They only have hyd. power in the UP direction. Gravity and weight pull them down.
You might try installing the rock shafts and lift links and try standing on the rock shaft for added weight to pull the hyd. lift arms down. As already mentioned, be sure the rock shaft drop rate valve under the operators seat is open to allow the rock shaft to drop down.
When all else fails, apply a lot of judicious force. So, I put the rockshaft control lever fully down, speed control to fast and then stood on top of it and hopped a little. 2 things happened - kids laughed, lift arms lowered. Everyone happy.