To tag multiple questions with categories, navigate to the Questions tab and select the questions you wish to tag. Click on Add Categories at the bottom of the page.
Clicking the expand icon will allow you to add more columns to the grid if there are any not being displayed.
Select categories by clicking the green plus button.
Click the box next to the category type to add them to the selected questions.
The selected category types will be listed with a search field for each. Search within each category type to select the categories to bulk tag for the selected questions.
Click Done on the top right once all categories have been tagged.