I am not sure why you would want to steam lentils. I make pilaf's out of lentils where I need to cook them aldente but I still don't steam them. As jennyema mentioned steaming would not work for a dry bean since it would stick and burn without any liquid (it takes a while for a steamer to produce the necessary liquid that would be needed to cook a dry bean)
I normally add them to water with some salt and let them cook until aldente ( Do not leave them and forget about them else you will end up with mushy end product).
Once the lentils are aldente, I normally strain them in a colander and reserve for the dish. You can use them in pilaf's, salad's etc.