Hmmm, here is what I think it could be.
You need an item/trainer card, and when you play it, you can search your deck for the mega card.
Example:
Lucario is on the field.
I play Lucarionite.
I get to search my deck for Mega Lucario, and get to put it on/evolve my lucario.
You need an item/trainer card, and when you play it, you can search your deck for the mega card.
Example:
Lucario is on the field.
I play Lucarionite.
I get to search my deck for Mega Lucario, and get to put it on/evolve my lucario.