Sightseeing in a class Bravo airspace?
Letters of Authorization (LOAs) are authorization to conduct various types of commercial flights in certain airspace.  They are defined and governed by FAA Order 8900.1 and can be obtained from the regional Flight Standards District Office (FSDO) for that area.  It is not necessary for non-commercial general aviation operations.  I suspect (and I don't know the full details as to how you contacted Ft Worth Approach) that the controller thought you were a commercial operation requesting to start doing some sightseeing over the Dallas Ft. Worth Area without having done this paperwork.  

Usually as a GA pilot, you can simply contact approach control for a Class B airspace and request a transition through Class Bravo.  This request may or may not be granted depending on commercial traffic congestion that day, etc.  They may also want you to stick to existing VFR traffic transition routes and not deviate from that.  Class B, as a general rule, is not very accommodating for sightseeing and other leisure flying.