All sites show 'invalid' – followed your instructions to a tee.

1) Installed domain mapping

2) In mapping configuration I put the server IP address

3) Went to registrar, altered DNS settings so the @ points to that IP address

4) Added the domain to the multisite client admin interface. It says invalid

5) Confirmed that domain points to that IP address.

Do I have to do something in my plesk control panel as well?