The stem is the piece that inserts into the movement through the case. Did you lose it??
It's late, but if you do a quick search of my posts, you should see me giving install advice to about 3 other people over the last 2 or 3 weeks.
The modification is the easiest mod known to man.. It sounds complicated, but it's not. Buy a new tube, Crown and stem from Ofrei or Cousins UK and the install takes next to nothing. You need Araldite epoxy and a good vice or clamp to put the watch in overnight while the epoxy sets.