If I remember correctly, Project+ only works for retail discs. Just extract Project+ to the root of the SD, insert your SSBB disc into the Wii U, and launch Project+ Luancher through the HBC.
I've done it another way since my Wii U is PAL, I've used an NTSC WBFS split file of the game, bought a new USB that is compatible (and has enough space) with USB Loader GX on it... and I've succeeded 👍🏼 Thanks !