MacArthur made his international debut playing for Team West at the 2002 World Under-17 Hockey Challenge, which was held in Manitoba. Two years later, he was invited to attend Canada's junior team Summer Development Camp in August 2004. He was ultimately chosen to the squad in December to compete in the 2005 World Junior Championships. During the tournament, MacArthur scored four goals in six games, tied for tenth in tournament scoring. One of the players he tied with was American Phil Kessel, who would become teammates with MacArthur in the NHL. In Canada's first game of the tournament against Slovakia, MacArthur scored two goals. Canada eventually captured gold at the tournament after routing Russia 6–1 in the final.