@ saikee:

Wow, great job! Thanks a lot for your explanations, all of my questions are answered now. And I guess once more it has been proven that a good example explains more than hundreds of pages of theory.

@ Todd50:

Yes, this is it. Whenever you want a new entry for a new distro you installed, you just click the Add button in that interface. In the Window for the new entry you can either directly point to the kernel image (the way I suggested in my howto without reinstalling GRUB) or enter the simpler commands saikee explained (in case you did install GRUB to the / partition of your new distro).

If you prefer to work with a texteditor (for example when there's no GUI available and you are restricted to the shell), you can edit the relevant file in the /grub subfolder of the /boot folder you pictured above.