Peter> >I've installed NetBSD 1.5.2 on an old ibook (The first version i think),
Peter> >and the installation went fine.

Peter> >But I can't boot it... I read something on the net about using Disk-Tool

I don't know about the Disk-Tool. Isn't this from FWB stuff, is it ? 
When you say "I can't boot it", it would be more informative if you tell what
boot command you type and what the OF show the problem.

Peter> I was advised to write to this list. I hope you can help me. I've come a bit
Peter> further, which means that I now know that I have open firmware 3, and I
Peter> think that Disk Tool wants to be on patition 9.

What happens if you type on OF ...

Did you got OF ?
As you said installation went fine, you should know this. But I am not sure.
Power on with command + option + O + F bring you to the OF.

Type at OF prompt,
boot hd:,ofwboot.xcf

what kind of messagess you get.

---
Makoto Fujiwara
http://www.ki.nu/software/NetBSD/iBook2/