Ok, before you try burying the stem any further...
1) Make sure your lights stay within a couple inches of the top of the plant. You dont want it reaching out for light any further than it needs to.
2) You can buy small bamboo stakes at your local hardware/gardening store. Stick one into the soil, roughly 1-2" away from your stem. I usually take a razor knife and make a notch for my string to catch on when I tie it off... thin string, small coated wire, twist ties, ect... you want something that you can tie off thats not going to be abrasive to the stem.
If a couple weeks passes, and you dont think that the stem is getting any stronger then you can mound up a small amount of dirt (it should look like a mini mole hill) around the stem. You just want enough to support it until it gets a little bigger.
Hope that helps.