When you first installed Slates in Facebook, one Slate is created for you automatically. To add new Slates click "Add New Slate" link on your Slates home page.

To get to the Slates home page any time, just click on the icon in your Facebook applications menu to the left of your screen. There is also a "breadcrumb" trail of links that's always available as you drill down into your Slates.

Adding a New Slate to the Slates Home Page

In the center of the page, look for the "Add New Slate" (pictured above) button, and click.  Name your new slate, add some tablets, and you are off and running.