When driving on the ring-road in direction to Hella, turn left on the road 26. Continue ahead till the big "Icelandic Horseworld"-sign and take the drive-in to the left pointing to Skeiðvellir. Park in front of the paddocks and the stable and enter through the door with the "Welcome"-sign outside.
You can cancel up to 24 hours in advance of the experience for a full refund.