To setup the syncing account for the Mac app, click the settings gear icon and open the preferences panel. From there, you will be able to enable syncing and will be given the option of creating an account or logging in to an existing one. Please note: you need to purchase MiniNote Pro for syncing (the free version of the app doesn't have syncing support).
For iPhone and iPad, you can open the side settings drawer and setup syncing from there.
For Windows 8, open the settings charm and sign into syncing.