Well, we didn't develop Horizon. The reason we choose not to troubleshoot on it is that we wouldn't want other people trying to troubleshoot our tools. That said, you have a few common options. Try opening the program as administrator (right click). You could also try a different USB drive, as sometimes it isn't formatted by the 360 as best as it could be (the 360 will tell you that it works but isn't up to standards). Finally, try reinstalling Horizon as administrator (again, right click).
Now, this is just common "everyday" advice for every editor, and not specific to Horizon. If none of these three things fixed your problem, then you really should keep trying to get that registration fixed. If you still don't get the email soon, try registering again; if it tells you that the email address you're trying to register under is currently being used, then that's how you know that you've used the right email address. Other than that, it's just a waiting game. It can sometimes take several hours, depending on the website (I once registered somewhere where I didn't receive an email for five days).
Of course, I wouldn't give up hope just yet. We have a wonderful in-house AIO editor that does most of the same things Horizon can do. Check out XB36Hazard's
Xbox 360 Tools - Infinity, and it may be a great alternative to getting Horizon to work. XB36Hazard is very talented and I use Infinity myself quite often. I'd say the only advantage Horizon has is that it's more popular; Infinity does all the same stuff and is just as reliable.
EDIT: Also, try to be a bit more respectful. There are reasons we aren't helping; it isn't that we can't, it's that we won't (the tool wasn't developed here). Even if we did develop the tool, nobody has to help if they don't want. And, chances are, when you take an attitude like that, people won't help you.