Also, when in Calibre, click on "Edit metadata" and then on "Fetch Metadata from Server." This will show you several entries for the same book. Make sure you check "Automatically download the cover." In the latest version of Calibre, you will see a check mark if the version you chose has a cover.
If you don't like the cover that is downloaded, just remove the IBSN number and try again. It is also worthwhile to to sign up for the free account at isbndb.com.