MY FIRST QUESTION concerns the part that says, " . . .that will work in full at all FSDT airports, and with a couple of default FSX airports."
I'm sorry, the Trial description on XPOI is not correct, that would be valid for GSX instead.
XPOI Trial is different, because it's not a product that has any relationship with a specific airport so, the Trial version will work for 30 minutes at once, while in an area 20-50 NM around several airports (doesn't matter if you installed a 3rd party addon for them or you use the default one).
A menu when you start it will indicate which areas are enabled for the Trial, offering the option to jump there immediately.
Does this mean that, before XPOI will work on my system, I must FIRST have installed a particular DreamTeam product? Is there some sort of base installation that I need before XPOI will work for me? Why will it work at only "a couple of default FSX airports"? I currently have no DreamTeam airports installed.
No, that would be GSX. If you own an FSDT airport, you can use GSX FOR FREE (without purchasing it), just install it. The purchase of GSX will allow you to use it everywhere else.
ALSO, I have FSX through Steam. Does XPOI work with the Steam version of FSX?
Yes, all our products do.
If someone could tell me everything I need BEFORE I install XPOI, I would be most grateful.
Well, the point a Trial is that you don't HAVE to "wait to ask someone", to check is something is working on your system...when you can simply install it and test it yourself.