Hey, so I had to do some trouble shoting to get this done. So, if you follow Scottkbender's instructions, you'll be doing good. However, if you find that you get a pop--up saying that you cannot edit this file because it's locked, follow these basic instructions:
1) Try messing with it in order to instigate a pop-up asking you if you would like to duplicate the file. Click duplicate.
2) In the new file, go to the Text Edit preferences, then "open and save."
3) Check the box under, "when saving a file" conserning adding the .txt at the end, in not already selected. Then, close out.
4) Make the changes specified by Scottkbender.
5) When you save it, you have to get rid of the original genres.plist file (hopefully, you copied it somewhere else or didn't empty your trash).
6) Your new file should be labeled as "genres.plist.txt" once you've made your changes. Now, click command + i to get "get info". Then, rename the file to erase the ".txt" at the end. Once you close out of the "get info" window, it will ask you if you want to use ".txt" or ".plist". Choose the latter option.
7) Open itunes and voila!
*NOTE: Scottkbinder's 18th step is not necessary.