Click on the "Download WatchFace" button
You will be redirected to download the PBW file.
At the top left of Safari you should see the "Open In". When you tap it you will see a list of apps, choose the "Open in Pebble" option.
N.B If the file size shows 0 kb try tapping refresh to force it to download
After you have done this once it should then show "Open in Pebble" in the top right.
It will open the Pebble app and ask if you want to install the file. After you tap continue it will load the watch face to your pebble!