2009 F750 not going into gear.

Have you tried easing up on the clutch gentley while putting it in 1st?

Sounds like its just a matter of the transmission internals being slightly off of alignment.

 

I suspect the clutch is dragging.
Does it grind going into reverse?

 

X2 on the clutch not full releasing and dragging.

 

If it didn't do this before the new clutch, then it's obviously the new clutch. I would bring it right back to the mechanic and have it fixed.

 

If you turn off the engine can you shift it into gear? Any gear. If you continue to hold the clutch down and start does it seem to pull the truck at all?
Some hydro clutches are like fighting a mother bear to get the air out of them.
Things like a disc not sliding easily on the input shaft can also cause this.
Would have to have some pics of the pressure plate to possibly commit on adjustment.