It just means that the installer is unable to add Megasync to E's menu and/or mimetypes....it might show up in Cairo-dock tho.
- The installment should finish despite of that error. If not, mention it to the creator as that would be a stupid restriction.
Meaning that you might need to run MEGAsync from commandline or search with Alt + F2.
Alternatively you can create your own icon on the dock by creating a .desktop in /usr/share/applications/ as I've explained as part of the tutorial here:
