Stewart signed on a three-year deal after his contract that was due to start with Leeds was cancelled following a loan spell.
Speaking to the club's official website, McCarthy said: "We've got good wide players and Cameron adds to that.
"He can play on either flank but he's a right footer who likes playing on the left, where he can cut in and look to shoot.
"He's very quick and can put crosses in and when we played against him when he was at Charlton and Leeds last season, he was one player that we highlighted that could cause us damage.
"We paid special attention to him and he's got that bit of quality about him. He's an exciting player who likes to take people on. He has the ability to make things happen."