Firstly you need to go and download an ftp programme such as Cyberduck, Yummy Ftp or Transmit, so that you can access the old iWeb site files on your server/host or wherever you hosted your old iWeb site?
You need to log into the server and then go to your Public html files on your host and delete any old iWeb site files that you find on the server.
Also, just so that you realise, Wix is not a web hosting company - if you use Wix, you can create a website quickly and it is one of those companies like WordPress.com, where you can create a free website that is hosted on their own servers, so you have your name/followed by wix.com. If you want to use your existing domain name with Wix, then you can. To do this, you need to go to your domain name registrar and ensure that you have used the Wix namservers where your domain name is. Are you sure that you have done this properly?
You need to check that you have forwarded your domain properly to Wix or inserted the wix name servers and then you need to delete your old iWeb site files. However, if you have done the first properly, then only your Wix site should show up without a problem.