Without having watched the video myself, have you initialised the 'aggroRange' variable in the create event 'obj_enemie'?
So that would be:
aggroRange = 100
(for example)
If you did, you might have set it to be a string instead of a real number?
If it's still not working, you might wanna set 'aggroRange' as a global variable in some controller object to be sure it really exists and can be easily accessed (and changed) by all instances.
Also I agree that you will propably get the best help on this topic at yoyo's own forum.