Go to the organization page. That page will show a list of buyers and what each buyer buys, like the one below.
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Find your name in the list. You might want to use the filter on the left to narrow down the list of buyers.

Once you find your name, check the list of brands that are currently assigned to you. If you see any brand missing in that list, click the edit action which will take you to the screen with your assignments. On the assignments screen, you can either edit an existing assignment or add a new assignment by clicking add assignment  button.

In the add/edit assignment  screen select the brands you are buying, the genders you are buying for and categories and hit save. If you are buying different categories from different brands, you can add them as different assignments.

After you hit save, you will see the selected brands added to your list of assignments.