Keep in mind that no work is ever actually done at the campaign level, so things like Time and Expense entry, Tasks, Assignments, Files, Diary conversations, etc, are all tracked at the project level. In light of this, some users have created a "campaign project" where this type of data can be used as a repository for the campaign.
Projects can be added at any time, even Closed projects are able to linked to a campaign.
Ways to link a project
There are 3 methods for adding projects to a campaign (see screen shot examples below).
Link existing projects to new campaign
Click icon located along left side to search, and select specific projects that already exist.
NOTE: The above screenshot shows the Use Multiple Segments options checked, with projects nested under each segment.
Create a new project from an existing template
From the bottom-right corner, Project Templates area, drag a template to the left. This will auto-create a new project using the selected Template. Client for the new project will be based upon the client specified in the Campaign setup screen.
Link the existing campaign from the project side
From within an individual project go to >Setup>General>Campaign: search for and select the appropriate Campaign. If the Campaign is setup with Segments, this field will be available once the campaign is selected. You must select a Segment.