Are you on a Mac?
If so, either via a chmod command in terminal (the below example assumes the file is on you Desktop, and is called Model.rvt);
cd ~/Desktop
chmod 644 Model.rvt
Or you can accomplish the same thing via finder - select the file, then Cmd + I (or File > Get Info).
Make sure the file is not marked as Locked (checkbox), and in the Sharing & Permissions sections, you should be listed as having Read & Write access to the file.
Google can help with more details, but here's a good start for you: http://www.macinstruct.com/node/415