How to use LT Artillery?

[Jingo]

I have a Long Tom Artillery piece I purchased in my MechHQ / Megamek merc campaign. How do I use that on the actual MegaMek battlemap? I assume someone needs to spot for it and then it shoots on its Indirect fire stage. But the Spot button doesn't seem to be come enabled for other units. Not sure what I'm missing.

[Hammer]

The coding for Arty follows the rules starting on Pages 179 in Tactical Operations