Here's a short tutorial (should take 30 seconds at most) to help you add more options to your dictionary. The default is based on where you live in the world, but you can add other languages as well as set your English dialect preference (American English or British English). Also note that I'm using OS X Mavericks and the screenshots below reflect that. I've also enabled file extension views so you see my application file names end in .app while your file names will likely not have extensions. Just ignore that and proceed. 🙂 )
Step 1: Launch the dictionary application (it's in your Applications folder).
Step 2: Select Preferences from the menu and view the pop-up list of additional options.