Create your campaign in the campaigns tab, be sure that the proper account has been assigned to the campaign.

Select accounts, then use the search bar to navigate to the account where the posts you want to add were published.

Select the appropriate social network from the drop down.

Scroll to the bottom of the page to see the Table View, containing all post types for that channel.

Choose the "Multi-Assign button" in the upper right corner of the table to add the posts to a campaign. 

Select multiple posts by clicking the "assign" button next to each post. If you make a mistake click "Unassign". 

When you have finished selecting all the posts you want to add, go back to the top of the table and choose the "+" sign. This will allow you to pick a campaign. 

Select the campaign from the drop-down menu. A success message will appear. To add posts to more than one campaign, select another campaign from the drop-down menu.

If you do not see the campaign in your drop-down check the campaign’s Start and End Date. The post must fall within that date. 

You're all set!