To add a Slate to your Audition use the regular iPhone camera to record in portrait.
Then go to Settings > My Profile > Manage Slates and import the Slate. Slates can be imported as portrait or landscape but can only be 15 seconds or less.
You can store as many Slates as you like.
To add them to your Audition choose a Slate in the New/Edit Audition screen.
A Slate can be added to the beginning or end of your tape.
Make sure to always follow casting directors instructions to the T with your Slate!