Make sure you save the file as a real Excel 2010 xlsx. But in my case I found my file was 'Shared' and removing the 'Shared' status solved it for me. Here are the two solutions in more detail:
Convert to xlsx
On the top of the Excel window, to the right of the filename, if it says [Compatibility Mode] do the following:
Click File | Info | Convert
Save the file as xlsx (native Excel 2010 format)
Turn off Shared
On the top of the Excel window, to the right of the filename, if it says [Shared] do the following:
Click Review | Share Workbook
Click Home and you'll see the buttons are now active!
NOTE: You have to make a choice! If you received the file via e-mail then it should not matter that you have turned off Sharing. But if you opened the file from a shared network drive then you will stop it from being shared - other users will not be able to access this file until you've finished with it.