There have already been posts on this. Mauer has never in his life played in a major league game, hence he is not yet activated to the major league roster.
Don't worry, Yahoo will switch it once he plays his first game. Just plug him into your lineup as normal - the stats will count even if you see the N/A next to his name.
"One of the teams represents truth, justice, the American way, and underdogs everywhere. The other represents George Steinbrenner!" - U.S. News and World Reports columnist John Leo on the difference between the Mets and the Yankees.