My fix for this was incredibly simple. I used YAWMM-Mod (not just YAWMM) to uninstall and then install the wads. Then the error didn't come up and the DLC songs are playable
The Mariocube tutorial says to use Wii Mod Lite, but that ended up being the issue.
Give that a try and let me know
I am in the exact same spot as you (just hacked my Wii, and love rock band games) and have the same issue, if you find a solution I'd love to hear it. I also followed the Mariocube tutorial and I'm dying to get the DLC working. I also see the red bar and then get the exact same message "The...