Mac Issues with CSV Files

If you are unable to retrieve any shipping rates, and are using Excel on a Mac can you make sure you save the files in Windows CSV format. This is a known issue, and documented by Magento for tax imports.
You will be alerted to this issue as when the upload happens it returns the number of rows uploaded. If this is zero but no other error is mentioned then it is going to be this problem.
You can optionally look in the table where the csv file is uploaded. The table name is likely to be shipping_<extn_name> e.g. shipping_premiumrate