In the preferences of the addon, you set the path to the Instant Meshes executable. For example in my case: C:UsersFlatronDownloadsinstant-meshes-windowsInstant Meshes.exe. It could be, after installing the addon, you need to restart Blender. In my case it was need. As last, to use the addon, select a mesh, press F3 and type: instant Meshes.