[chevelle]

The way it works is....
The shifter pawl & plate work to select the gears.
The gears have dogs (4 or 5) that engage into the slots on the other gear.
The gears must be turned to sync the dogs into the slots.
This is why he said to try and turn the rear wheel, which will turn the main shaft and the gears will mesh.

 

[chevelle]

If it is a cross over (right to left) 75-76 bike like mentioned, I believe that shaft is splined to the inner lever.
Not sure what controls your using but be sure all your shift pivots/ levers are situated correctly and they do not contact any parts that would keep the shaft from rotating far enough to engage gears. m