See the video above for Lightroom Classic CC installation instruction which begin at 2:30 in the video.
If you plan on using your XMP presets with the Lightroom app on your mobile devices, then the Lightroom CC installation is preferable. The instructions for a Lightroom CC installation are listed at the top of this page.
1. Start Lightroom Classic CC. Select help > updates and ensure your Lightroom Classic CC has been updated through the Adobe Creative Cloud. Older versions of Lightroom Classic CC will not permit XMP preset installations.
2. Open the develop module and ensure a photo is visible on your screen (this is important).
3. Look for the word "presets" in the top left corner of your screen. Beside the word presets is a small + sign. Click it.
4. Select "import presets" and select the XMP presets file you previously downloaded. It should be located in your downloads folder. Select your XMP presets download file and click the import button.
5. Your installation should be finished. You will see the presets folders appear inside the presets panel, below the word "presets". You may have to click the little triangle to the left of the word presets. If your presets do not appear, restart Lightroom and check again.
Special note to Mac users... sometimes Mac computers have a setting turned on that automatically unzips any downloaded zipped files. Lightroom is only searching for a zipped file, which will explain why your "import presets" button does not appear to be working. You can either re-zip the presets folders and try to import them again, or you can copy and paste them to the following location:
Macintosh: HD\Users\<username>\Library\Application Support\Adobe\CameraRaw\Settings