I am on a Mac and I have downloaded the licensekey.dat file but my server is not accepting it. How can I fix this?

Apple Mail detects ".dat" files as "embedded text" or metadata for attachments. When you open the file the first 3 lines should look something like this:

company name : First Last
address : 123 Main Street
zipcode : 12345

Populated with the information you provided during registration. If the first 3 lines of your license does not look similar, you will need to download the file using WebMail or something other than Apple Mail (eg - Thunderbird).