The Calendar folder should be in the Library folder which is under your User's folder.
In the Finder open the Go menu, press the Opt (⌥) key and select the Library folder. You should see the Calendars folder in there. Open that in Time Machine.
Remember I am restoring from my time machine back up on an external drive. So when I go to back ups I'm on the external drive and not in the finder. The library folder is visable immeditely and there is no calendar folder in it.
Sorry, from what you said it sounded like you had Time Machine set up. The point of Time Machine is that you should not need to root around on the external drive. I'm suggesting you open your User's Library folder in Time Machine. See: http://support.apple.com/kb/HT1427
If you cannot use Time Machine to access the folder and you need to do it manually. Go into Users > YourUsername > Library to find the Calendars folder.