The authentication log in as plugin will be automatically installed and enabled with UE Man (v1.3.0 or greater). This plugin must remain enabled for you to use the log in as feature.
To turn on log in as, go to the component options of User Enhancement Manager
Components->User Enhancement Manager
Options button (top right)
User Tools tab
Set enable log in as to yes, and select the user whose password will be used.
Make sure that the user you select to have this super power has a strong password