The Categories section is found in the left side panel together with the other main sections in the software.
In the Categories section, click on + New Category to create a new category.
Then you can name your category and choose the color of its button in the Button Color dropdown menu.
The name of the category and its button color that you can set here also sets the category button's appearance on the directories. You can see the button's preview at the end of the button color field.
Once you have created your category, you can then assign your offers to it by editing your offers.
Once you've assigned your offers to the category, you can now filter the offers in the directory using the category buttons. When you click on a category button on a directory, the offers that are assigned to that category will be hidden from the category - you'll notice that the button shows an "x" before the category title. Clicking on the same category button again will unhide the offers that are assigned to that category.