I just finished installing Ubuntu on a Windows Secure Boot system. I noticed during the install, a black screen with "Binary Whitelisted" written on it. The normal 12.10 version was used on a Flash drive.
But right now, it looks like you need to get into the boot devices menu. There should be a key option for your machine. How those keys work might depend if you are starting from a cold boot, or a restart. But I did see one thread that said hitting F1 repeadly during the splash screen would get you into the bios. Possibly there you will see a boot device option, or be able to set the Flash drive as primary boot device.
What model Lenovo do you have? I saw the medion reference, but we probably need a full model number.
Edit: I also found this thread which is very recent and might help.
bios - LENOVO desktop computer only boot with F12 - Super User