1) Click 'Manage Categories' under the 'Lists' heading of the 'Control Panel'.
2) Under 'Existing Categories:' select the category you'd like to add keywords to from the dropdown menu.
3) Click the 'Edit' button
4) Scroll down, next to 'Keywords', enter the keywords you'd like to add seperated by comma.
5) When you are finished, scroll down and click the 'Save' button.