To use Direct Mode, just press F10, hold down CTRL and use the cursor keys to select the tile you want to use. Make sure you're in editing mode 1 (platforms) - even if you want to add background tiles. The best thing to do is get the screen looking as close as you want it before going into Direct Mode, because once you do that, you'll have to make every change on your own, and turning off DM, to the best of my memory, will destroy your changes.