These instructions are based on the Snow Leopard UI, but they should be similar in other versions of OS X.
1. Follow the directions above to enable the Character Viewer.
2. Open Character Viewer and in the top list, scroll down to 00002300 Miscellaneous Technical.
3. In the grid below this list, find line 2310. You should see the Enter, Option, Command, Tab and other symbols in that general vicinity.
4. Make sure your I-beam text cursor is positioned where you want the symbol to go in your document, and then double-click on the symbol in the Character Viewer.
Note The font you are using may not have these symbols. Try using Lucida Sans (the system font up to Lion) or Lucida Grande.
Tip: With the symbol selected, open the "gear" menu in the bottom-left corner of the Viewer and choose Add to Favorites.
You can also then use the Symbol and Text Substitutions table in the Text panel of Language & Text to automate entering those symbols. Once that's done, you can choose
Since I've just done all of this, I'll paste what I've found below:
⌘
⌥
⌃
⇧
⇪
⇥
↩
⌫
⌦
⇞
⇟
↖
↘
Hope this helps.