Log into your account and go to the Campaigns & Bid By Domain section on the left menu.
To pause an Entire Campaign, click on the green Active box next to the Campaign Name that you want to pause and select Paused from the drop down menu in the Status column.
To pause a Single Ad within a campaign, click on the Campaign to view the content within. Then locate the Ad you want to pause and click the green Active button in the Status column. Select Paused to pause the content.
To Un-Pause content at either the Campaign or Ad-level, click the gray Paused Status button next to the Campaign Name or Ad and select Active.