The first thing to do is to ask what guide you used to mod the Wii? A lot of times a bad mod will cause a game to fail to load. The second thing, if you did indeed use our guide, would be to say check this guide out:
That should help you get things going. I found it after 30 seconds searching our forum with the search being "guitar hero 3 not working".
If I spelled something wrong, I'll just blame it on auto-correct