The priorities tab exists within the SEO dashboard under the GWT: Top Queries tab and Pages tab.
In the GWT: Top queries tab, scroll down until you get to "GWT Top Search Queries List". Under the "All" tab, you will see all of your queries. At the right side of the screen are check boxes you can use to mark certain queries as priority queries.
In the Pages tab, you can make different landing pages or exit pages priority pages by using the check marks on the right side of the screen.
Once you have marked certain queries or pages as "priority" you can easily see them in the "priority tab" as shown below.