Well, I'm gonna take a wild guess here
If you're using GUI, you might wanna try this out:
Start by making a region or any size, and put the unit inside the region you want for to wander.
then, Create a simple trigger -
Events : Time - every (5.00) seconds of game time (or any amount of time you see fit)
Action : Unit - Order (unit) to Move to (random point in region(your region))
now, every few seconds of game time, the unit will select a random point in the region and move towards it. If the region is big, you might want to make the event time a tad longer too. Experiment with this until you are satisfied but that's a way to make a unit wander (in the case if the unit has no other purpose than to wander.)
If I'm not mistaken, there's also a passive unit ability named "wander" or something, it's the ability that critters in the game make use of to wander. but If you want the unit to wander in a controlled area, I suggest you keep to this trigger
I'll attach an image for a bit more clarity.
Good luck and hope I could have been of service ^_^