Try NS-USBloader:
https://github.com/developersu/ns-usbloader
It's suppose to work with Tinfoil as well.
Run the program, go to settings, then click on
Download and install drivers. If
"allow XCI / NSZ / XCZ for Tinfoil" is unchecked, check it. Open the first tab again, select the the files, then click to upload (although it won't actually upload it, but allow you to open it in Tinfoil and install through USB).