If it doesn't make sense to you then this is what you do. Firstly, make a unit variable and in a trigger set it to a unit. Then in the trigger where you want the unit to perform the animation add a .1 second wait time so no other animation overrides the one you want it to perform. Next add a custom script like so.
As I've shown, you put the unit variable name in the paranthesis then a comma and then the animation integer of the animation you want it to perform.
To get the integer use something like WC3 Viewer. When looking at the animations of a model in the viewer remember to go in numerical order top to bottom counting the first animation as 0. In other words if it looked like this:
None
Stand 1
Stand 2
Stand 3
Attack 1
Attack 2
Say you want the unit to perform attack 2. Counting from stand 1 which counts as 0 you reach attack 2 as the integer of 4. So for that example, say your variable is Person then this is how your trigger should look at least.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.