hi, is the brake pushed down and locked; is the PTO switch in the off position (pushed down); fuses all operational? with a fully functioning machine, you would need to be sitting on it provided the seat switch has not been bypassed. once the machine starts, to engage the deck, you would pull up on the pto switch(the yellow switch) on the right of the panel. it is also referred to as the rio (reverse implementation) as you must pull up on it prior to mowing in reverse, otherwise, the deck will disengage.

I assume the engine is running.
Pulling the PTO switch does NOTHING.

Check to see if you are getting 12V to the electric clutch.

I don't have a schematic for that machine, but sometimes the GROUND circuit to the PTO is also switched on/off by an operator presence relay. Possibly one that has been disabled by jumping the seat switch? (sometimes they serve TWO functions)

Without a schematic however, I can only speculate.