http://forums.cnet.com/5208-6121_102-0.html?forumID=45&threadID=22053&messageID=356012 wrote:
Subscript (Letters/numbers typed below the line of text):
After typing CO (or whatever text you want), immediately press the Ctrl key and the equals (=) key at the same time, release the keys then type your number 2. It will become subscript with the number two lower than the preceeding letters. To get back to the main text line, press the Ctrl key plus the = key again.
First result I found in Google. Go figure.
You can also do it through Format->Font IIRC