1.) tick the user account you want to allow access
2.) put the username in the "Additional Users" box at the top - i.e. next to the DefaultHost (nobody) entry
3.) DON'T tick the exclude protection box for the DefaultHost (nobody) entry
Apache's mod_userdir allows users to view their sites by entering a tilde(~) and their username as the uri on a specific host. For example http://test.cpanel.net/~fred/ will bring up the user fred's domain.
This can be useful particularly when waiting for DNS propagation to complete however opens your server to potential abuse, in particular bandwidth abuse which cannot be tracked correctly in this case.
I would suggest enabling the protection in WHM and only excluding trusted users.