If im not wrong, the RSE can only be activated if u bought the full version instead of dling the free demo version.
As for the RSE sounding awful, i feel that it's not the problem of the RSE. Rather i feel that it's a volume balance problem. Because most tabs are "mixed" in midi, and the some of the RSE sounds are relatively softer than the others, so when u turn it on the sound balance is just not right.
Another gripe is the drum sounds. I feel that the snare and bass drum is too soft. Kinda give me headaches whenever i play songs from GP4/3 tabs.