You will see a hole that holds the spring. It may be dirty- filled w/ old anti- seize. The spring goes below the key and the key is held down as the gear is slid on. Use a good coating of grease on the shaft.

Thank you tomplum very much. I had to dig really hard to get that dirty, rusty little spring out of that hole in the drive shaft. It all worked out - again many thanks. Now we will see if the engine will start later.

Found out that the front drive - I knew it didn't work - has bad gears in the transmission. Ideas on where to get cheap parts - and the complexity of repairing the transmission.
Thanks..

Well, it's like this. I usually replace the tranny if they want it repaired. ~ 60$ if I remember correctly. You may even be able to source it cheaper on Ebay. Just get the proper #. A poster here before couldn't figure out his drive issue after installing a transmission- and he was sent a rwd trans and his was a fwd unit- and it turned backwards!
I believe that Toro services the gears and bushings separately- which are the right parts you need. It isn't a hard repair, clean up is the worse part. There wouldn't be much inside other than the parts described above and a gasket, possibly seals. You will want to know that the shaft isn't worn where it is supported by the pivot arm and of course that the bearings there seem fine.

Here is a link that might be useful: Toro parts and manuals