When you finish creating a campaign, there are two possibilities: either publishing it immediately or just saving it.
After finishing the editing of a campaign, click on the button Save and Publish on the bottom of the page. It will lead you to a page where you see in which players the campaign is added. As you have just created it, there will be no players on the list. The campaign is already saved on your account, and you might leave the page if you want to publish it another time.
If you want to set the campaign to a player immediately, click on the Publish to Players button. Fill the settings, such as to which players add the campaign, when and where to play it, etc.
For publishing previously saved campaigns, you may go to the Campaign tab and click on the Publish button under the desired one. Then you apply the same steps as if you had just created a campaign: click on Publish to Player, fill the settings and hit the Publish button.