Hello Jessica, this is a real joke since yesterday all days long I tested and retested the room reservation email to see if buttons worked and never saw them. This morning when I get back to the office, zam! buttons are there!
I am still so disapointed of Outlook for Mac 15 since it's a 100% improvement over Outlook for Mac 2011, but it's still so less reliable than Outlook 2013 on Windows.
This kind of things never happen on Windows and the room reservation approval worked right away when I tested it on my Windows 8.1 PC running Outlook 2013. But unfortunately this supervisor is using a Mac... so... had to live with it.
Thank you very much for your support and thank to Mouran too. You can classify this bug as "IT mystery" than only happen on Mac!