In order to add user imported data to your campaign, the campaign type needs to be user import. You can either choose the campaign type while creating the campaign or change it retroactively as follows:
go to campaigns > open the campaign
go to settings above
the right side panel will appear
click the slider button on the bottom to change your campaign type for imported contacts
last step is to click on save changes
Please note: You can only change the campaign type before adding the prospects in your campaign and before starting the campaign.