My my my, now don't we have the first excelent question on this board, I'm currently looking at the custom AI scripts, but the easiest way would be to fake an AI script by having a swordload of triggers that resemble <trigger 1> at the bottom of this post.
<trigger 1>
Events: <Unit- is brought to location>
Conditions: None
Actions: <If <Looks like time to attack=True> Then <If <Randomized Boolean= True> Then <Attack Area 1> Else <Attack Area 2>> Else <Patrol away>
((In other words, path out a route for the enemy to take and just have them do that, And have I mentioned I LOVE If/Then/Else))
<Added section through the post edit>
Alright, I have now looked at this, and understand why Blizzard does not support the use of this editor. AI's are stored in .ai files that must be made with some other editor- one that blizzard has not released due to instability problems. Your best bet (if you wish to actually make these scripts) is to look for an AI editor on either at
www.warcraft.org or the website refered to in the tread "How to get Starcraft units in Warcraft"