Pretty sure it is in game.txt, n'est pas? The search function there makes that file more user-friendly.
Also be aware of the oddities with using this pop up for certain situations. For example using the %STRING# in the pop-up sometimes presents the wrong STRING information. That is to say, if you have only one missile unit, try something like "UNITNAME attacks rain death from above..."
I have seen, more than a few times, the STRING data being set to something very wrong, usually the defending Civ's leader name. This results in alot of
"Winston Churchill attack near Portsmouth!!!" kinds of pop-ups when the Germans launch their V-1's etc...
Therefore, I prefer to set it to something more generic, not using the STRINGs at all. This way your pop-ups are always correct, and if you have only one missile unit, you can replace its name an not sacrifice any particularities in the message. You will of course loose the "attack near...." part of the message, but since the map centers on the missile unit and you have to click 'ok' to allow the attack to progress, you dont really lose any details in for the region the attack is taking place in either. Just a few thoughts there.
Do as thou wilst shall be the whole of the law, however.
