create a locator, and have the model follow it. if you try to key the root joint to move, it might effect the other joints in an undesired way, and gets confusing to keep track of. but if you make a locator that the root joint follows, it makes things a lot easier.
create>locator. it'll appear in the project window. it helps if you place it right above the model, so it's easier to keep track of. you can also make more than one, for example if one locator is keyed so the model walks along the x-axis, and another locator is keyed to move it up and down, like for stairs.