You can pause or activate your campaigns at any time within your account.
Pausing a Campaign
From your Dashboard, click Manage-> Campaigns.
Check the box next to the campaign you want to pause.
Then click Actions-> Deactivate.
Click "Yes" on the confirmation box that pops up to deactivate the campaign.
Activating a Campaign
From your Dashboard, click Manage-> Campaigns.
Click the "Inactive" tab.
Check the box next to the campaign you want to activate.
Then click Actions-> Activate.
Your campaign is now active.