I just saw that CotW had gone on sale on Desura when I got home today, so I rushed to buy it. However, now that I've bought it, I can't actually install it on my 64-bit system, as it gives an error saying "Conquest of the Wizardlands is not supported on this platform", and there doesn't seem to be any option to force it to install. I'm assuming that CotW is like the other expansions, in that it just involves a folder full of XML files and images, without any actual 32/64-bit malarkey. Hopefully it's just that the wrong checkbox got set somewhere in the process of uploading to Desura, and it'll be quick to fix.
Oh, well that sucks. What exact problem are you having, the same as me? I was able to purchase the game fine, and it showed up in my games list, it just wouldn't let me install. It definitely told me I was buying a Linux 32-bit version, I just didn't think that was going to cause a problem.