For objects:
1) Trial and error in the Mission editor..
2) setpos command:
this setpos[getpos this select 0, getpos this select 0,x]
x = height above ground
for units:
1)make waypoint directly over name of enterable building in ME .... you'll know you've done it right if you cannot move the waypoint after....
most all enterable buildings have position numbers.... after placing the waypoint on the building you should see another field come up.... "position number" (or something close) ;D
and you can select a number there..... and the dude will go to that spot in the building