If you want to use mods on a Mac make sure you follow these steps because Mac's merge files differently than windows.
1) Download a new version of Tennis Elbow for Mac, http://www.managames.com/tennis-mac/game.html
2) Download whatever mods you want (The Maxou Patch is great)
3) Extract the files from the downloaded patch
4) Hold down the Option key and Drag the folders (Data, Menu, Etc...) from the Mod to the Original Tennis Elbow folder
5) When you see the prompt "An item named Data already exists...", click Keep Newer. If Keep Newer is not a prompt, make sure you are holding down option. If you are holding down option and your only you only see Replace or Replace All, choose Replace All
6) Enjoy the Mod
Hope that helps