There is a another way of controlling this using back-end procedure but it is not recommended because this feature is already in-built within Apex.

Setup
APEX: 3.2.1
Web Server: Apache (OHS)
Database:

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Prod
PL/SQL Release 10.2.0.3.0 - Production
CORE    10.2.0.3.0      Production
TNS for 32-bit Windows: Version 10.2.0.3.0 - Production
NLSRTL Version 10.2.0.3.0 - Production

First I fired up the web server:

C:oraclehttpopmnbin>opmnctl start
opmnctl: opmn started

C:oraclehttpopmnbin>opmnctl startproc process-type=HTTP_Server
opmnctl: starting opmn managed processes...

Opened up APEX, and created a new application. For authentication schemes I chose "No Authentication."

After I had created the application, I went into Shared Components –> Authentication Schemes –> Create

Select the default and click Next

Select "Show Login Page and Use LDAP Directory Credentials" and click Next

I’ve already done this so I’m selecting my current Login page, 11, click Next

Enter your LDAP Host and your DN:

Your DN String should look something like this (from article above):

cn=%LDAP_USER%,l=amer,dc=oracle,dc=com

Make sure you use the %LDAP_USER% after the cn= portion of the string.

Name it ldap_test, click Create Scheme:

You will then be redirected back to the list of Authentication Schemes, ldap_test should now be current

To test it just run your application and login using your LDAP (AD) credentials

Success!

Thanks to http://www.oraclenerd.com/2010/03/apex-ldap-authentication.html