If you mark the box, "In the future, do not show this message", the message advising you of the last time you backed up your company file won't appear.
If, after choosing to not show the message, you wish to have the message appear again, you can manually turn the message back on. Select Preferences from the Setup menu and then choose the Security view. Simply mark the selection, "Prompt for Data Backup When Closing".
As often mentioned, we at MYOB UK strongly recommend that you back up your Accounting Plus information regularly-and very often.