- Merlin set it up all of those years ago before he re-took human form, to have Vala find the tablet in order to lead her to the cache of treasure and the communication device in order to lure the Ori to the Milky Way. The person Vala stole the tablet from was one of the descendants from the "Knights of the Round Table". Signs to watch for had been passed down over the years and the tablet was made to be as enticing to Vala as possible, to lead her back to Daniel. Merlin knew Daniel when he was ascended and that he'd return to our plane of existence and used that, used Daniel since that was the only way to destroy the Ori. What brought her to Merlin's attention was Qetesh and Athena's search for the Clava Thessara Infinitas.
(from Stargatewiki about Athena: She was searching for the whereabouts of a device called the Clava Thessara Infinitas (Key to Infinite Treasure) which had supposedly been left by the Ancients before their ascension. This Treasure was later deemed an elaborate hoax, but Qetesh had led Athena to believe that she had found it.)
- The Clava Thessara Infinitas (Key to Infinite Treasure) is actually the Ark of Truth. Because truth is power and power is treasured. Both truth and power can be warped to suit ones needs. (if you look at the picture of it in Momento Mori, you can see that it has a lot of similarities to the Ark.)