Working trigger, copied and pasted
Area: 4x4
Activation Box: anybody present
Name: trig14
Condition: this and onMine && "land" counttype thislist > 0
On Activation: grenade14 = "grenade" camCreate [getpos trig14 select 0, getpos trig14 select 1, 0]
onMine is a variable which switches the minefield on and off. It is true when East units are in the general area, and false when they are not. This is to prevent West units from setting off the mines when East (the player) is far away and thus "wasting" the mines.
There are much fancier mine scripts out there, but this does the job for me. I have some variations: sometimes the grenade is created underground to simulate a sort of "toe-popper".
On Activation: grenade14 = "grenade" camCreate [getpos trig14 select 0, getpos trig14 select 1, -12]
Sometimes two grenades are created, which is a bit nastier. ;D Sometimes the grenade is created in the air, falls to the ground and then explodes. This gives you a split second to duck if you see it, although there is no warning. (The minefield is in a forest, so its simulated the grenade falling out of a tree. It's a bit crap but variety is the spice of life, or at least the Un-Testable Mission.)
You could of course create the grenade at the position of the loon who set off the mine. Syntax not guaranteed but try something like
grenade14 = "grenade" camCreate getpos thislist select 0
This would guarantee a very severe case of death, whereas putting it at the centre of the trigger gives chance of wounding.
To learn scripting generally, read both snYpir's Friendly Intro to Code Snippets and also Johan Gustafsson's Scripting Guide. You can find very simple working examples of scripts in the Tutorial Mission. All in the Ed Depot - tutes - getting started.