We found this too, when we were trying to get some practice for DS last year.
What I have to say won't make the manufacturers happy but....someone let him do some on their PC and frankly you aren't missing anything. DS is very computer savvy so I thought that route would be better for him but he actually preferred the written ones and I was glad I had saved my money. The exams are written, so they might as well practice in that format. Unless someone knows an easy route, I wouldn't stress over it. That said, most schools-type stuff is Mac compatible; the only problem I have ever encountered was (sorry EPE, in every other respect you are wonderful) trying to buy stuff from this site.
And Macs just are
so much better, aren't they?