Thank you Cameron,
Before posting here I did a search on Google and found the same solution as you did, but it did not work, so I posted my problem here. This alert generally says that a dmg file is corrupted. Though, I could successfully open another dmg file on my MacbookPro. As a test, I redownloaded the update 1.08 on a windows 7 partition on my Macbook (which I use for FSX), copied the dmg file back to my OS partition and got the same error.
I also created a dummy X-Plane folder on my Win7 parttion and successfully installed the 1.08 update to it. Do you think I could just copy those files (Aircraft - Resources/plugins - X-Aviation) back to my X-Plane folder on my Mac OS partition?????