Here's a duplicate of my answer in the other thread:
If you are only trying to do this from the helm, you probably should check and see if you can get it into forward at the gearbox (i.e., reach down and pull the lever up and down through all three settings F/N/R). If she goes into gear, then you know that either your cable or control lever is the culprit.
If you can't get her into gear at the box, then somethings amiss internally.
Like Mark mentioned, if you can't get it into gear manually, then a repair
is likely needed. The good news is that the ZF10 is a light box, so once it's free of the engine and the shaft it won't be a bear to get off of the boat. Depending on your shaft/prop/rudder set up, you may be able to do this without hauling the boat.