This video will show you how to manually add a person into a campaign without using the stage moves method: https://youtu.be/XVd74p_kiEE

Please note that if someone is already in a campaign that is active or paused, you will need to unsubscribe them to perform this.  This IS shown in the video above, but important not to miss otherwise this process will not work.

If you are uncertain about how Stage Moves automatically ads leads into campaigns, it will be worth reviewing that as well (because you may actually need to do this instead of a 'manual add'):  https://youtu.be/U2F5MjGPTxY