First thing you'll have to do is upload the file to an internet available server. You can usually get around 5MB of storage free with your ISP (Internet Service Provider). You will have to get an FTP (File Transfer Protocol) program. You can download these for free, or I could e-mail you one. This allows you're PC to transfer data to/from the server.
Once you have the file stored on the internet server, then all you have to do is copy the URL (link address), and paste it into the "link" box that pops up when you click on the link icon. See the pic below.
Hope this helps. Let me know if you want the FTP program, or more help.