When getting it from the download page make sure to right click on the link and "Save Target As . . ". Don't copy/paste the XML file, that file is a uuencode'd file, you need it in it's correct format.
At least that was my mistake (trying to copy/paste). The error you're making might be different, as the symptoms may just be an indication of the potions file failing to work.
AlaCarcuss: Hmm, not working for me.
I'm having the same problem as SlackerSupreme, only I've tried everything and it still doesn't work. I've tried all the files linked in the first post and save them directly to CockedPC\items. In the case of the 30min one, I of course had to rename the Items.zip to def_stats_item_potion.xml (not sure why it's a fake zip file?).
I even tried using different browsers, Firefox, IE, Chrome, not that I think that make any difference. The symptoms are identical to SlackerSupreme - i.e. no effects showing at all, though they are removed from inventory after drinking.