Ok, the above is all correct (see also the Respawing sticky thread) but they missed a few tiny vital points / hints that might help you out.
1: Every line in description.ext MUST end with ;
If you don't do this, weird stuff happens. Empty lines don't need a ;
2: Respawning does not work unless you are in MP mode. That is, click Multiplayer, create server, select your mission etc. Don't bother Previewing to test respawn. It won't work.
3: You don't have to export it to MP missions. If you Copy your mission folder and put it in Users/YourUsername/MPMissions, it will appear as a yellow mission named "your folder name" when you start a server. You can Edit it from here too. I'm unsure whether respawn works if you do this, I've never tried it, you may have to export to test that.
If you're editing MP missions, you can do it via the Multiplayer menu if you've moved it. This lets you be different playable units easily, but it takes a lot longer to preview, since you have to select player etc & go through the briefing and intro (if you wrote one)