Scenery Loading Query

Hi I have many sceneries ORBX etc etc some freeware some payware, when I want to load a scenery for a flight do I click on add on scenery in FSX and load it from there as there only seems to be a few listed?My ORBX ones aren't listed in the add on tab but are loaded in the scenery editor section, so do I just pick Melbourne intl and it will load ORBX files?Also does the AES file need to be at the top?Or does the scenery you wish to run need to be at the top of the list?Thanks for your help in advance?Wayne H