User with dba privs can't login to APEX, but can from SQL*Plus I'm using Oracle 11gR2 XE. A user with dba privileges can't log in to the APEX interface, but can from the command line with sqlplus.

I just created user:

CREATE ROLE administrators;
GRANT dba TO administrators;
CREATE USER superuser IDENTIFIED BY "root";
GRANT administrators TO superuser;


If I run cmd, type sqlplus and log in with "superuser", I get access.

But the web interface (APEX) denies access with an "invalid credentials" error message.

What do I need to do to be able to log in on the APEX interface?
The APEX administrator accounts are not linked to a database schema. Reciprocally, creating a database schema won't create an APEX account.

You'll have to create the administrator account via the web interface. 


Connect to the administration page:


  
  If your setup uses the embedded PL/SQL gateway, go to:

http://hostname:port/apex/apex_admin

  If your setup uses Apache and mod_plsql, go to:

http://hostname:port/pls/apex/apex_admin

  


Then On the Login page:


  
  In Username, enter admin.
  In Password, enter the Oracle Application Express administrator account password you specified when you installed Oracle Application Express.
  

There are three levels of administration, decide which type you want to create:


  
  Developers create and edit applications.
  Workspace administrators are developers who also perform administrator tasks specific to their workspace such as managing user accounts, monitoring workspace activity, and viewing log files. See "Workspace and Application Administration".
  Oracle Application Express administrators are superusers that manage the entire hosted instance using the Oracle Application Express Administration Services application.
  



I suppose you want to create an APEX administrator account. They are workspace administrator of the special workspace "Administration" that you can create following the standard APEX account creation.