Your questions are vague but let's see how I understood them...
> How do you chose where art things appear.
Well, it depends just what you want to do. If you wish to attach an effect to a unit, look for Art - attachment points under the buffs editor. You will need to add that buff to the unit somehow (auras work well) and in the buffs section add effects and specify where they are going to be attached with Art - Attachment points 1/2/3/4 fields.
> And is is possible to make a art (without modelling) come out of the ground and be thrown at the enemy.
To expand on that, if Wratox1's explanation for what you mean is correct, you could indeed do as you stated there, and the way that I personally would do it is a flying dummy unit with a model from an ability/projectile that starts with a negative height, and use triggers to adjust height/distance, easily creating the desired effect.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory
I will be AFK for a couple of days you guys hold down the fort while I am gone. I will be checking in on my phone but that will severly limit me. Be back saturday!