Well you create a dialog per trigger:
- dialog - create trigger
then you save it in a variable e.g. "raceChoser"
Now you create the buttons for it:
- Nightelf, orc, undead, human, e.g. naga....
and save EACH one in a variable -> "butNightelf" "butOrc" ... and so on
Then you create a trigger:
Event: Button is pressed for dialog
Condition: -
Actions:
IF(pressed button = "butNightelf")
-> Create nightelf starting units for triggering player (I think triggering player will work here)
IF(pressed button = "butOrc")
-> Create orc starting units for triggering player
and so