I had this same problem with Yosemite. As it needs driver signing, you must override that.
To do so, boot with the kext-dev-mode=1 argument.
@Emily, can you sign this driver so we don't need to do this?
If you have clover, add the following lines under your "Boot" config area:
<key>Arguments</key>
<string>kext-dev-mode=1</string>
If you have Chameleon or one of its spinoffs, open your /Extra/org.Chameleon.boot.plist file and make sure it has something like this in it:
<key>Kernel Flags</key>
<string>kext-dev-mode=1</string>
If not, add it. Save and reboot.