Mitch,
Do what Cap'n said. Just take a crow bar or spade shovel and separate the ground in multiple areas around the tree and place the moisture mizer in that opening and use your heel to close it back up. If we get the MM below the ground it will not run off but will go downward toward the roots.
If you are concerned - do a smaller circle and then a larger circle around the seedling.
Trust me - moisture mizer is going to help the seedling. Do an experiment in your yard. Find a small depression where it can't run off and place a teaspoon of moisture mizer on the ground in that area. The next time it rains go look at it. If you don't want to wait on the rain, use a hose pipe after 2 or 3 hours - spray it fairly good. Maybe not instantly but in hours you will be the expansion properties of the product ( the ability to retain water). It looks like a water blob or jelly. Great product that does as advertised.
Good luck and Merry Christmas teach.
Wayne