Thanks Steve. I was following step by step the tutorial I mentioned above and this onehttp://maketecheasier.com/install-dual- ... 2012/08/27
that reads "On the rEFit screen, press the right arrow to select the USB installer, and press and hold the “Ctrl” button while the USB installer is booting up. (Note: Failure to do this step will result in kernel panic)"
The Ctrl key doesn't work. After my last message I tried the F6 key and it worked. The 'e' key identically works too. After adding the noapic option, LMDE booted and ran. But LMDE can't find my internet connection even though I'm connected with an Ethernet cable.
A first outcome is rEFIt might not be needed any longer on the new Macs running Mountain Lion since the Apple EFI seems to allow to boot other OS than Mac OS now.