If you only have one terrain the easiest way to address it is to use an action with a pointer:

ENTITY* my_terrain = NULL;

action act_my_terrain()
{
my_terrain = me;
}

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terrain_min and terrain_max are meant to be the position of the lower left and the upper right corner of the terrain respectively.
I am not 100% sure but I think terrain.min_x/max_x works:

vec_set(terrain_min,my_terrain.min_x);
vec_add(terrain_min,my_terrain.x); // my_terrain.x may be 0 because I think the terrain gets converted to world coordinates
vec_set(terrain_max,my_terrain.max_x);
vec_add(terrain_max,my_terrain.x);

If this does not work you will have to figure out a way how to set those vectors up. You can do it.