What is the file extension? If it's .dmg, then select the downloaded file and Control/Right-Click it and select 'Get Info'. Look for "Open With" (you may need to click on the triangle to reveal the info). In the pull-down menu, tell me if you see DiskImageMounter (linky)?
You can also mount the downloaded file (if it has the .dmg extension) by launching DiscUtility (found in the Utilities folder in your Apps folder).
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.