To add new keywords to an existing campaign, find the campaign you wish to add to on the Campaigns page.
Moving your mouse over the top-right of the campaign graph will highlight a shortcut to 'add new keywords to this campaign'.
You can also find the 'Actions' button located at the bottom-right of the campaign graph. Choose 'Add Keywords' from the drop-down menu. You will be re-directed to the campaign setup page, where you can enter additional keywords to run prospecting for.