[lubuntu] IMac: Problem installing Ubuntu so... Lubuntu?

Hello everyone!!
I have an IMac with a 32 bits processor and I can't install Ubuntu on it (I don't really know why, when I press ALT, the dvd is ejected) so I was wondering if it's possible to intall Lubuntu on my mac and if so, how to? If not, who wants a mac?
Thanks in advance!