When you've completed the business, bank and employee setup tasks, the panel containing the Signup button will no longer be available. If that happens, you can still sign up by selecting at least one super payment on the Pay superannuation page and clicking Pay super. In the popup window, click Go to MYOB's super portal.
You'll be able to sign up from there.
Note that you'll need to have completed at least one pay run that included a super component since 1 July.