Only a few nations are able to get any megaspells at the moment, those being Red Eye, the Fillydelphia Contingent, Tenpony Tower, and New Appleloosa (though they only get it briefly).

-Balefire Bombs are activated the same way vanilla nukes are, only you don't have to have air superiority.

-Healing Megaspells are activated through a unit leader ability (like Last Stand, etc.) if you have Waking the Tiger enabled, or by a button in the megaspell menu if you don't

-Celestia One is exculsive to Tenpony Tower, and has a special GUI that you access by clicking on a state once you've unlocked it.

Update to that, with 1.2 it is now possible to obtain a Balefire Bomb or a Healing Megaspell via the scavenging decisions (very, very small chance, and only certain states). Still activated the same way.