Cannot install newest version of Parallels Desktop for my Mac, please Help!

This is the support request I tried sending to Parallels Desktop but apparently I have no support! Please could you forum people help to resolve this issue, I would be very very happy! Here is the message I sent them:

Hi. When I try to install Parallels desktop I am presented with the message 'A new version is available: It is recommended to install the new version of Parallels Desktop 4.0. Do you want to install it now?'. Once I click 'Install current', the following message appears; 'Unable to start the installation: The downloaded DMG file cannot be mounted. Go to your Mac Desktop, mount the "(null)" file manually, and start the installation again.' I have mounted this 'null' file and tried to install but am still getting the same message! This has happened every time I've tried installing it, I can't even use it the software after paying out for it!

Any ideas? Or anyone had a similar problem?
Thanks!
Amie