Glad we could help a new player.
Press the U key for a list of creatures (that's the general DF term for 'living things') in your fort area. Take note that this list can become extremely long as a fort ages, as deceased creatures still also remain on this list, but are moved to the bottom part of it and marked as deceased. A slight annoyance is that this list doesn't support zooming the view to a selected creature's (corpse) location if that creature is deceased. (of course, corpses tend to get into bits and pieces scattered all over the place)
Another thing that should help you a lot is the anouncement screen (press A), if you don't know of it yet. A message in bright pink/magenta should be in there announcing the death of your dogs. Since the latest few versions, you can select an announcement and press Z to zoom to its location of origin. That should help you figure out where and when the dogs died.
Lastly, a huge help is the combat announcements screen (I think that's its name). Press R to view detailed accounts of recent voilent festivities in and around the fort. A red C or somesuch character should show up on the screen frame on the left edge, almost at the top, whenever there are new combat announcements. This should help you figure out exactly what killded your mutts, if it was a wild/hostile creature or a violent fall down a shaft that killed them (though the latter is usually very unlikely).
Now btw, being someone quite lazy who has got to know the vanilla interface quite well (it's all I play with, and it works for me), I wouldn't exactly know the answer to this, so I have to wonder out loud whether there isn't a convenient single place where nubbies can go and learn the interface basics.
And heck, where useful things to know are made obvious, like the fact that you can change Z-levels with Ctrl+5 and Shift+5 (that saves me a lot of arm motion as opposed to using < and >).