This is a known issue, and due to a problem with a Windows update when they moved the openCL dll. AMD, Microsoft and NVIDIA are all aware and working on this.
The Mettle files are being installed to the correct locations - they simply don't appear in any of the options either in the effects or transitions window, or in the Playback preferences.
For example, if you are running Windows 10, with an Nvidia GTX1070 graphics card, with 32GB of RAM, and both an SSD and a M.2 PCI-e 3x4 SSD for storage and playback, HUD is the Oculus Rift CV1.
1. Make sure to download the latest driver from your GPU vendor.
2. Then hold down the shift key next time you launch Premiere Pro or After Effects.