I got it installed. At least the most information I got from here:
What I did:
installing rEFIt, which is pretty easy. I had the problem that rEFIT didn't start without pressing [alt] while booting, but the solution is the manual install which is easy and described in the rEFIt documentation.
create a free partition using bootcamp (on snow leopard)
boot an ubuntu dvd. You can do that pressing [C] while booting.
Probably I wouldn't have had so much trouble when I skipped the bootcamp part and just let ubuntu split the thing automatically (it seems like ubuntu can do this, i didn't try).
But I have to warn people. On my macbook6.1, about 4 Years old, I can use macOS 4 hours and more, but ubuntu only 2 hours until the battery is empty. Even with using powertop, jupiter etc. Many power saving tutorials do not work anymore because they are too old.
To improve Battery life try things like proprietary GPU drivers, laptop-tools-mode and so on. Also there is a repository called mactel on launchpad, providing applesmc and macfanctld which will provide a better automatic fan control with good standard settings, easy to edit.