Both the FSDT and the default AFCAD files for KLAX have parkings with A and B so, if you don't see them in FSX, the most likely explanation is that you have another scenery (non default) in conflict.
The most common cause of conflicts, are scenery files supplied with AI Traffic packages, like MyTraffic, Traffic X, etc. Those scenery should be used only if you have the default airport installed, but must be removed if you have an add-on airport.
You can use the free FSX Airport Scanner utility, available here:
http://www.scruffyduck.org/simple-airport-scanner/4584282795To find for duplicates on an airport. You should only have the default one, and the one that comes with FSDT KLAX. Any extra must be removed.
Also, be sure the KLAX airport is on top of the Scenery Library, which is where the installer place it by default. If you move it manually, you increase the chance of potential conflict so, as a general rule, the add-on airports should always be on the highest layers.