Apparently the Windows version has triggers, but not the Mac version. I have used PowerPoint on a Mac through many versions and I have never seen the trigger button on a Mac.
Both seem to have all the other options, Delay, Speed, Repeat, Rewind (with animations other than Appear), but no trigger option on the Mac.
Let Microsoft know you want this option. Maybe in their next update for Mac, which should be around 2012, they‘ll include it.