You can pause or activate your campaigns at any time within your account.
Pausing a Campaign
From your Dashboard, click Manage-> Campaigns.
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Check the box next to the campaign you want to pause.
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Then click Actions-> Deactivate.
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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.
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Check the box next to the campaign you want to activate.
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Then click Actions-> Activate.
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Your campaign is now active.