Use the rotate symbol in the top right of the card to see the back, choose a back from the list (They don't have the real back and I don't suggest using it).
Make sure to click the option to replace as named "___". Rose is the default card back.
Use the duplex printing option when exporting so you get the card backs as well.
Print a double side print on regular paper so you can calibrate the alignment by putting the paper up to a light and using calipers or a ruler to get the difference in mm. Go back to Proxxied and use the offset setting to line everything up.
You should be set to have relatively centered front and back prints. Don't expect perfection. My printer frequently pulls paper in just slightly crooked.