Dosen't work on 2.76b. Even tried manually loading the plugin and it won't see it.
Secondly, it keeps telling me that there are over 1000 vertices with zero weights on them. I'm thinking that if that was the case, then I'd notice.
Try mine: https://github.com/motorsep/blender-idtech4-md5
I still need to add a tutorial, since it requires a few setup steps to export properly.
The gist of it is that you have to have 1 root and call it "origin", each animation has to be in a separate Action, use local markers on each Action to define length of animation, when you are done with your rig, select all bones in Pose mode and initialize the add-on by clicking Init button in the Properties tab (you can select any bone after that and use checkbox to mark it as exportable or not exportable bone; there is also an option for re-parenting bones on export, like original Doom 3 Maya plugin did).
Being in the Pose mode, select all meshes attached to the armature, and then select any bone in the armature (meshes have to be selected first, bone - last). Go to Export menu and choose MD5 format. UI is pretty self explanatory.
I really should make a video as when you see it, it won't look as complicated as it might sound
Note that your deforming rig and your meshes you are going to export have to be on the same layer in Blender. Bones can be scattered on any layer in bone layers.