However, as far as I can tell, it is dependent on a database server connection which goes against one of Microsoft's biggest database project selling points - disconnected development.
If you're like me and haven't been able to find this functionality I included a screen shot below to help you find it. The key is the Server Explorer tab.
- Create a connection if you don't already have one.
- Expand your connection and right-click on Tables -> Add New Table.
Once you have defined your table you can use the Table Designer - > Generate Change Script . . . menu option to create a DDL script. If you save the definition it will actually save it to the database.