When selecting a session category, if the category you want is greyed out and not selectable it means you need to affiliate to that business.

To do this go the Affiliations page in your admin portal. The first page shows you all of the businesses that you are already affiliated to. To affiliate with a new business, click on to the Affiliate tab, find the brand you want to affiliate to and click the Affiliate button below their logo. You will need to agree to the terms of the affiliation set by the brand but once you have completed this your affiliation will be complete and you will be able to select any categories pertaining to that brand in the session and course category fields.