To make this easy you should download and install the latest version of forge (if you haven't already) you can get it at files.minecraftforge.net. I suggest getting the installer for the version it is. Then start minecraft you may have to reenter your username and password and make sure the profile is "forge"
Step 2: Download Mod
Go to www.mccapes.com and at the bottom of the page it should say download now, click that and download forge - MCCapes "extra" mod. It should say the version it is meant for.
Step 3: Install Mod
Navigate to your .minecraft folder if you are on windows vista or newer search %appdata% in the start menu if your computer is older than vista seach that in run I don't know how to do it on Mac. In the .minecraft folder there should be a folder called mods drag and drop the cape mod file into there.
Step 4: Select Cape
Go back to mccapes.com and in the box at the top enter your minecraft username (your username not your email) and press enter then enter a password for that site and then go to cape gallery and chose a cape. form there on it is self explanatory. Press use this cape and then save option.