Installation problem

Okay. This keeps happening, and I don't know why. I'm following my host's tutorial on installing this bulletin board. I've added a MySQL database and added a user, both with the exact same name. I downloaded and unzipped punbb into the same folder all my other site programming is in. The name of the folder is the same as the name of the database and user. I uploaded the punbb folder to my FTP. The tutorial is telling me I should be able to go to the installation screen, but I keep getting an error 404 message--i.e., the internet can't find the site. Please tell me you can help me with this.