They made a lot of changes between 5 and 11 or 12--whichever you are now using. One of the biggest changes is that you now can have multiple tables in one file where you had to have one table per file in version 5.
To define a new field or modify an existing definition:
Select Manage | Database | Fields and you'll see a window for defining fields much like FileMaker 5, but with a drop down for selecting the table added.
Enter Table View and click the + --- I'm not a big fan of this option but it is a way to add a new field if you have the DB open with a full access password.