Just because I am a dork, the above instructions weren't 100% clear, and it took me some while to make it work. For my fellow dorks, here is some additional details.
1. The Audio-Midi Utility is the one included in Mac OSX under applications>utilities (I was trying to find it elsewhere, like within Logic, or the Sound menu in Sys Prefs--ugh)
2. The "audio devices" tab must be highlighted, and the audio menu is up at the very top of the screen.
3. You must select BOTH the snowball mic (or in my case, the Samson CO1U) AND the Built-in Output. The aggregrate device didn't show up when I only had the mic selected.
I know this may seem utterly obvious to some of you, but hopefully it helps the rest of you!