You’ve said both:
“I should mention I never downloaded the Poser files for any of my content,”
“I did indeed download and use both installers”
Does that mean you have since gone back and downloaded the Poser installers you had ignored before? For Studio you need both the Poser and Studio sets of files. If you haven’t been back and rerun the installers, then you need to do that.
Despite files being referred to as ‘Poser’, Studio has always used them, it has a parallel set of files that can be used solely on Studio, but to allow content to be used on both Poser and Studio it has always been provided in Poser format (Generation 5 content for Genesis is the only exception). You can’t ignore the Poser files, they are fundamental to the products you are buying, and you need to be able to find your way around the Poser file structure - which is things like runtime/libraries/character when seen on the disk, or Poser Formats/(parent folder of runtime)/figures when seen from within the Content Library Tab of Studio.
“this item requires you to select an item in the scene”
That’s the error message when you try to use a texture without having the item it goes on selected. Are you getting to this texture file via the Content Library tab (specifically under Poser Formats/(whatever)/figures/m4cowboy), or via the Smart Content Tab? How many different files do you see there?
“The only thing I haven’t installed yet is the additional resource files that you can download from the product page. It’s asking me where I want to install them, and I have no idea what to tell it.”
Those are template files, they are used as guidance for creating new textures and are completely unnecessary for the outfit to function. They can be installed anywhere, or not at all. It won’t make any difference to the whether the outfit functions or not.
I’ve checked back and I do own the Cowboy pack. I’ve just reset it and there are four installers:
5205552 8/1/12 Cowboy For M4 and Hiro - Cowboy For M4 and Hiro - Poser Files - exe - pc Available 4
5205552 8/1/12 Cowboy For M4 and Hiro - Cowboy For M4 and Hiro - Poser Files - zip - mac Available 4
5205552 8/1/12 Cowboy For M4 and Hiro - Cowboy For M4 and Hiro - DS Files - exe - pc Available 4
5205552 8/1/12 Cowboy For M4 and Hiro - Cowboy For M4 and Hiro - DS Files - zip - mac Available 4
You need to download and run both of the files identified as PC, which I’ve bolded (or the Mac files if you are on a Mac) nb your order number, the first thing on each line, will be different, but that is not significant.
If I download and run the Poser files installer, 8648_4_dpc_CowboyForM4andHiro_4.exe then it installs 15 .CR2 files into runtime/libraries/character/m4cowboy, 4 .PZ2 files into runtime/libraries/poses/m4cowboy and 4 .PP2 files into runtime/libraries/props/m4cowboy (and other files elsewhere, but these are the important ones).
If I download and run the Studio files installer, 8648_2_ds_CowboyForM4andHiro_2.exe, then it installs 17 .DS files into (my content folder)/people/Michael 4/Clothing/M4 Cowboy, and 6 .JPGs into (my content folder)/runtime/textures/RD/M4 Cowboy.
You need to confirm that you have all of both sets of files installed, and run the relevant installers if you do not.