How did this unsigned driver get installed on Windows 7 64 bit?

You can disable Driver Signature Check enforcement as follows.Open a command prompt as an admin and type:

bcdedit -set loadoptions DISABLE_INTEGRITY_CHECKSbcdedit -set TESTSIGNING ON

See security risk warning.If it doesn't work for whatever reason you can just remove loadoptions with bcedit and switch testsigning off.

bcdedit /deletevalue loadoptionsbcdedit -set TESTSIGNING OFF

Goodluck!

By default Windows 7 is set to reject unsigned drives. But there are many inside out options that can change this property. One can change these properties unknowingly. Seems this is the case with your system.

For further reference see Windows 7 - Disable signature verification of drivers

Initial notification seen at the time of installation indicates that the driver is not gone through Microsoft windows hardware compatibility test. In x64, windows enforcing signed drivers is to check the integrity of the driver at load time. It doesn't validates whether the certificate is signed by a trusted root or the time stamp is valid. You can create a driver even with expired certificate. This driver is signed by a valid certificate which is expired. I used to signed my drivers with expired certificate by back dating the system time while signing & load it successfully. Code signing has to be done only with a list of vendor certificate, which is mentioned in microsoft site "VeriSign Class 3 Code Signing" is one of them.