This is a known problem for many drivers under Windows 7 64 bits.
Detailed explanation here :http://www.mydigitallife.info/force-load-unsigned-drivers-in-test-mode-easily-in-windows-7-or-vista-with-driver-signature-enforcement-overrider/
So you can install the driver by running the following in a command prompt:
bcdedit.exe /set TESTSIGNING ON
But then you get a "Test mode" text on your desktop. It's enough if you just want to dump some disks then uninstall the driver, or if it doesn't bother you to have this message printed all the time on the desktop