From what I'm reading, you want an area to be fired at by artillery when BLUEFOR enter?
http://community.bistudio.com/wiki/Artillery_Module has a lot on artillery.
How I'd do it (and there's probably better ways).
Set up your artillery battery, either virtual or real.
I haven't played with virtual, but for real:
place as many artillery units as you want in a group, and then place an ARTY logic module in.
Name your arty logic something (lets just say "artbatt" for this)
Put an invisible helipad where you want them to target (I think marks work for this too, but can't remember for sure, and I know invisible helipads do), name this something (lets say "arttarg").
Set up your trigger for when you want the artillery to fire. (I'm sure you know how to set the trigger up).
In the trigger On Activation, you want:
[artbatt, getposASL arttarg, ["IMMEDIATE", "HE", 0, 15]] call BIS_ARTY_F_ExecuteTemplateMission;
IMMEDIATE can be either IMMEDIATE or TIMED (IMMEDIATE is x number of shots, timed is x number of seconds)
HE can be:
HE = High Explosive
WP = White Phosporous
SADARM = Search and Destroy Armor
LASER = Laser Guided Arti Shell
ILLUM = Flares
SMOKE = Smoke
The first number (0) is time between shots, 0 being as fast as they can do it. (They will not fire faster than they can reload).
The second number is how many shells, or how long to fire, depending on weither you have immediate or timed.
I think that's right, I'm not at my comp with arma atm, so can't really check ><