You can rank up by doing well in Career mode. You get EXP for doing good things, like sweeping a series, winning 10 games, winning postseason games, and signing good free agents. You also lose EXP for doing bad things, like getting swept in a series or losing 5 games in a row. The EXP that you gain goes towards ranking up. You can view how much EXP you have and how much more EXP you need until you rank up by going to My Career. Use a good team like the Boston Red Sox or New York Yankees so you will be able to get EXP faster and easier because you will win a lot more.
You should easily be able to reach level 30 when you are going for the 30 year career achievement.