you need to go to the roster page, and slide the DL guy into the DL slot, if your league has one. There's 3 kinds of slot, bench, starter, and DL. Your league will have roster size rules. If you are at capacity for healthy players with an open DL'ed player, once you DL that player your team roster page will say at the top that you have an empty roster spot. At that point you can add a player.