Getting back on topic, as Xyon said most Orbiter add-ons (including the hi-rez textures you mention - which are higher resolution textures for the same planets and moons you already got in the Orbiter Base) are installed rather simply.
You simply unzip them into the "Orbiter Root Directory".
OK, maybe it's not as simple as that sounds. When I say "Orbiter Root Directory" I mean the folder that contains "Orbiter.exe". If you used the MSI installer package it's probably something like "C

rogram Files/Orbiter". If you used the Zip package, it will be whatever directory you extracted the .zip file into.
You need to make sure that you unzip the package with the right option - and what the option is called will vary depending on what Zip utility you use. In some utilities the option is called "Use Folder Names", and in other it is called "Keep Directory Structure Intact".
It's a good idea to make a back-up copy of your Orbiter install before adding ANY add-on package - that way if something goes wrong you can revert to the original easily.
Unlike most commercial software, Orbiter does NOT change the registry - so most add-ons don't need "intallers". This allows you to have more than one Orbiter Install.
I, for instance, have separate "Installs" for different purposes. One is a fairly "Clean" install, with few add-ons (mostly Orbitsound, IMFD, and the XR-2) which I use when making tutorials. Another is specifically for AMSO (a popular add-on for the Apollo missions), and one "General" install with quite a few add-ons installed.