Ahem,
Well, perhaps stuff could be explained a little better and more here, actually. Okay, so: to grab the "object" of a pre-existing building on the map, you can either use its ID number (as seen when 'Display IDs' is on), or use a command like
nearestBuilding. Using nearestBuilding, you could do it like this:
1) Place down a game logic right on top of the building in question. In its init field, write something like:
ObjectiveBuilding = nearestBuilding this;
Then, place down a trigger with the appropriate command:
!alive ObjectiveBuilding
Now try to blow up the building and see if it managed to "pick up" the correct building with the command
(add a hint or something to the trigger).
Should work!
Wolfrug out.