A campaign can be created and saved without any ads linked, however it must be linked to at least one ad before it can go live. An ad can be linked to any number of campaigns, and a campaign can have multiple ads linked. You can also link and unlink ads at any time once the campaign is saved to your dashboard, regardless of whether it is running live or not.
Linking a Single Ad from the My Ads tab
From the My Ads tab, select the ad you want to link. Click the grey gear drop down under the "Actions" tab and select Link Campaigns.
From the "Link to Campaigns" window, all your campaigns will appear under the Campaigns section to the left. Here you can select the campaign you want to link and select Add. The campaign will then appear under the Linked Campaigns section.
Once you are done linking the campaigns, click Save.
Linking Ads in Bulk from the My Ads tab
To link multiple ads to a campaign, select the campaigns you want to link and click Link to Campaigns.
You will be able to link the ads to a single campaign, or multiple campaigns, but keep in mind all selected ads will be linked. Under the Campaigns list, select the campaign you want to link, and click Add.
You can repeat this step for as many campaigns as you need. If you need to remove the ad from a campaign, simply click the X next to the campaign in the Linked Campaigns list.
Once you have linked all your campaigns, click Save.
All linked campaigns will appear on the lower half of the My Ads portion of your dashboard.
Linking Ads from the Campaign Editor
Head to the "Linked Ads" tab in the Campaign Editor, and click Link Ads.
From the "Link to Campaigns" window, all your campaigns will appear under the Campaigns section to the left. Here you can select the campaign you want to link and select Add.
Once you have linked all your ads, click Save.
All linked ads will now appear in the Linked Ads portion of the campaign editor.