One of the fields you can use to classify your database contacts is Contact Source. If you would like to designate a Contact Source not already available in your database, you have the ability to add new Contact Sources.
To do so, under your Manage Your Account menu, select Manage Database Lists:
The body of the Manage Database Lists page contains all of the lists that you can edit in expanded form. You can work with individual database lists by clicking their name in the Lists box to the right:
After you've clicked Contact Source List, your page will refresh displaying only this list:
You can add new Contact Sources by clicking the Add button in the top right corner. You'll then see the below dialog:
Just type in the name for your new Contact Source and click Save Contact Source. If you had added a new Source named "Seminar Lead", you would then see it in your list of Contact Sources, as below:
Contact Sources have both Edit and Delete options beneath them, meaning you can edit their name or delete them entirely.
After adding your Contact Source, it will then be available in the menu when editing a contact's Source.