Come on guy, all this stuff is on the forum, it's time to learn the search function.
Take all those folders out of texsrc.
Open only one of your skin folders and find the skin for it's exterior.
Name it properly. Might be b17g or b17g0 for instance, can't remember atm.
Put only that renamed file into your texsrc folder and rebuild your terrain. It will replace the default skin when you have the name right.
After you have that working, add another plane one at a time being carefull not to overload your terrains' size.