Follow these steps in order to install the MD5 export plugin for blender:

1. Download the plugin from here
2. Open Blender (The plugin has been tested on Blender 2.76)
3. Goto “File → User Preferences”
4. Goto “Add-ons” tab and choose “Install from file…”
5. Browse for the plugin zip file and press “Install from File…”
6. Search for the plugin you have installed “Import-Export: Export idTech4 (.md5)” and enable it via the checkbox
7. Press on “Save User Settings” and close the window
8. You will now see the option “idTech 4 MD5 (.md5mesh .md5anim) from the “File → Export” menu