Quick and Dirty Guide:
Installing Blender to Ogre Script in Blender

Step 1: Download necessary files

You will need to download two tools -- the Blender2Ogre plug-in script (to create .xml files that are compatible with the Ogre representation of meshes) and the OgreCommandLineTools (to convert the xml files into the binary file format that ogre likes

Step 2: Installing and activating script

Open up blender. From the file menu, select user preferences:
User Preferences

Select the Addons tab, and click Install from File

Navigate to the folder where you unzipped the blender2Ogre script, and select io_export_ogreDotScene.py
User Install12

Press the "Install from File ..." button, and you then have a huge list of addons you can install. Either search through all of these to find the Ogre one, or type Ogre in the search field, and the click the square to activate this addon
User Install12

You're done! Go back to the Quick and dirty blender tutorial and export away