Installation instructions (from the Urho3D Google discussion group)
> extract the folder 'io_mesh_urho' from the zip to the blender scripts/addons folder:
Windows 7 - C:\Users\%username%\AppData\Roaming\Blender Foundation\Blender\2.xx\scripts\addons
Windows XP - C:\Documents and Settings\%username%\Application Data\Blender Foundation\Blender\2.xx\scripts\addons
Linux - /home/$user/.config/blender/$version/scripts/addons
> start Blender
> enable the addon: menu File > User preferences > Addons > Import-Export > Urho3D export (tick the checkbox on the right)
> if you can't check the box, search the console for errors (menu Window > toggle System Console)
> you'll find a new panel in Render page (at the bottom) of the Properties window