Step 1: Click on your"Campaigns" in the upper left-side of your account:
Step 2. Click on the specific campaign you want to add the prospects to:
Step 3: Go into the campaign, and click the "Add Prospects" button in the upper right hand corner of the screen.
Step 4: You will then be redirected to your prospects table where you can choose one or multiple prospects from your imports and then click "Add to Campaign":