Ensim assigned them a Site# and called them Admin#. For example:
http://ipaddress/~admin# or
http://ipaddress/~site# http://ipaddress/~sitename.com would get you into their acct.
In Cpanel, you put their username only -- not the domain, admin# or site# like Ensim. With an Ensim client you gave them a user name to get their email with and they used that to access their email account. THAT is the user name you should use. For example
http://66.66.666.66/~mary/
This works for me. Hope it works for you.
Also rememer, you can only have ONE user named "mary" (example) on your whole Cpanel system.
caveat: I am a Cpanel noobee. I think I am right but defer to those with more knowledge --