Maybe this experience helps.
I was following the instructions exactly - except I had created the \quake and \quake\ID1 directories on the PPC not through ActiveSync. I won't bother you with why - but there was a good reason.
Anyhow, having got the error and reading the helpful guides here, I tried another approach - I created the folders directly onto the memory card (my PC doesn't have ActiveSync - but it has an SD card slot). Then copied the files in and replaced the card in the PPC. And it worked.
Now the interesting question - why ? When I now look at the directory names on the PPC they are in CAPS - even though I entered them lower case. So I conclude that the directories need to be created FROM THE PC for it to work. At least it is worth a try !
Hope it helps others avoid the frustration.