I am trying to log into the program, but it only lets me login with the SYSDBA. How can I login with another user name?

If EPIM tells that user doesn't exist and you are sure that username and password were entered correctly then most probably such user wasn't created or it's disabled.

Do the following to correct this:
1. Login as SYSDBA
2. Navigate to Tools -> Users
3. Create or enable necessary account(s).