Did you happen to add anything else? acid blend? nutrients?
What was the expiration date of the yeast?
What was the starting gravity?
What is the current gravity?
I ask because meads sometimes don't show nearly as much activity as beer when fermenting, and checking the gravity can confirm that the yeast are active.
It often helps, when using liquid yeast, to make a starter. The cell counts just aren't high enough otherwise, and can give a very long lag phase. Try aerating the must really well to help the yeast in there grow and divide more, and it might be helpful to add some nutrient. If it doesn't get going in the next 24 hours, you may want to repitch using a starter.