Ok, I've figured out how to install libtxc_dxtn under Ubuntu Natty.
Before we start, here's an optional part: How to add the debian-multimedia GPG public key to the trusted keys of Ubuntu's package managers. This prevents some warnings and error messages later.
Open a terminal and enter the following commands:
gpg --keyserver pgpkeys.mit.edu --recv-key 1F41B907
gpg -a --export 1F41B907 | sudo apt-key add -
That's it. It really shouldn't hurt to do that. Now for the actual installation of libtxc_dxtn step-by-step:
fyi: I just went over to google, typed in "ubuntu libtxc_dxtn" and read through the very first search result. Maybe you didn't get what to do there; it also took me some time to figure it out, so no offence.
Thank you very, very much, helped me a LOT! No more Wine, wuhey :p