First of all check the file size of LR, it should be around 419 mb. If not, then the download might have not been completed successfully. You might want to ty downloading the same again from a different browser.
I tried using Firefox, and that did the trick.
1. Download and install free preferencepane RCDefaultApp from http://www.rubicode.com/Software/RCDefaultApp/
2. Under tab Appl., find application DiskImageMounter
3. Check UTI "com.apple.disk-image-cdr"
This should suppress your problem.
I confirm. Using Firefox did the trick for me as well.