The simplest way to handle no-shows is to just wait an hour (or until time control) and give the opponent a forfeit win. In larger or more formal tournaments, that's what is usually done. This explains what to do if you would, instead, like to see that the players get a game.
In the first round, you probably should just match up the players without games. Usually, this is most easily done in the tournament room with the switches either handwritten on the results sheets or on some other form. How To: Correct Pairings describes how to transfer these changes into your Games Window. The Drag and Drop method is usually the most convenient way to make the corrections. If you create a "board" with two no-shows, entering it as a double forfeit makes sure that the two players are withdrawn from the tournament. If you want to, you can later delete the "game" and remove the players completely (How To: Remove a Player).
You can also use WinTD to redo the pairings, but it probably won't be worth the wait for the players unless it's a small tournament. Plus, the USCF rule book recommends against re-pairing anyone involved in a game where Black has made his fourth move. If you want to, though, use the steps described in How To: Redo Pairings. Make sure that you reset the Round to Pair box in the Pair a Round Dialog Box.