President Reagan is scheduled to nominate Ann D. McLaughlin, a career public-relations executive, as secretary of labor today at a White House ceremony, sources told the Washington Post. If confirmed, McLaughlin will succeed William E. Brock III, who left the Cabinet post this week to head the presidential campaign of Senate Minority Leader Bob Dole (R-Kan.). Neither organized labor nor conservative Republicans are expected to oppose the McLaughlin nomination. McLaughlin, 45, served three years as assistant Treasury secretary for public affairs during Reagan's first term. Later, she was appointed undersecretary of the Interior, a post she held for three years.