When you first unzip FuseBuilder and access it in your web browser, you're prompted for a username/password via the login form. Because there are no users recorded in the app at this point, any username/password combination you enter will automatically become the first User, and will be set as an admin.
After there is at least one user, you'll need to enter in the username/password correctly from then on to be able to get past the login screen. If you enter a combination that isn't found in your list of users you'll be returned to the login form. In no instance is it advised that you leave the username and password empty when submitting the form.
If you can't log in no matter what you do, you could try and open up "users.wddx.cfm" (found in the "/secured" folder off the fusebuilder root) in your text editor, and try and find the user from there. Barring that, as a worst-case scenario, just delete the "users.wddx.cfm" file and enter in the username/password you want to use as the primary admin's login.