Hmmm... what exactly is the problem? Under the "textures" menu, do you see a list of "level-ish" names, (bespin, common, doomgiver, system, yavin, et cetera)? And when you pick one of those options, does Radiant think for a second while it loads .jpg file after .jpg file, then give you a window full of textures to choose from? Have you changed your layout from the default four-pane style (thin z-slice on the left, large work space in the middle, 3D view on upper right and textures on lower right)? If you changed your layout, perhaps you have lost your textures window. I'd suggest just getting used to the default layout, it's not that bad once you figure out the keyboard shortcuts to change the cycle the 2D viewport. If none of this is the cause of your problem, I'd suggest you re-install Radiant. Don't try any custom crap, just let Radiant install how it wants to. It takes about two minutes to install. Then try your textures again.
By the way: there is no need whatsoever to unpack your assets0.pk3 file. Radiant can read it. Something else is the problem.