How To Install Addons...
1. Open Blender & select the "File" menu.
Under "File" select "User Preferences". Select the "Add-ons" tab at the top of the user preferences window then select "Install From File" at the bottom of the window.
2. Select the right file type.
Here's the tricky part. Some addons must be installed by directly selecting the Python (.py) file while others can only be installed as a zip file. If you don't find a folder with "__init__.py" inside the downloaded package, and instead find something like this "someSweetAddon.py" then it means you have to unzip it first so you can select "someSweetAddon.py".
3. Browse through your computer.
After pressing "Install From File" you'll be taken to Blender's file manager where you can browse through your computer's folders to select the file you need (be it someAddon.py or a .zip file). Once you've selected the right file press "Install From File" again (this time in the top right corner).
4. Check the box.
Once installed, check its box to activate it. Press "Save User Settings" near the bottom left of the User Preferences window before you go. This ensures the addon will stay activated.