Getting a job as a game tester requires three things: a strong desire to play games for hours at a time, written communication skills and contacts at a gaming company. Game testers need to play parts of a game long enough to break it and also write about the process.
Game testers often play particular games or even game levels over and over until they find everything that could go wrong in the game and report it back to the game designers. This requires a specific skill set that is generally honed by years of game play leading up to their employment as a tester.
They are also fairly active in gaming communities, often posting on forums or writing their own blogs. Many of them are beta testers for the companies they work for and post on the official community forum for the company.