Normally if you have Sync With User Management enabled in Admin->iSymphony V3 in FreePBX, the user credentials are defined in Admin->User Management. I am not sure how these fields work when using Active Directory. One option is to disable Sync With User Management and all user credentials and properties will be managed under the iSymphony Settings section in Applications->Extensions->[ext].
You can take a look at the following page for more information on these settings:
http://docs.getisymphony.com/display/ISYMDOCS/FreePBX+Module+Administration