Sitebuilder Plus is an online editor. If your account is still set to Sitebuilder, then you will need to call the support line to have your account updated to Sitebuilder Plus. We do recommend that you download the Mac version of Firefox as your browser.
Actually, I've updated your account so you can try using Sitebuilder Plus. You still have access to the desktop version. If you can let me know if you still need the desktop version that would help. We don't typically recommend using both since some elements available in Sitebuilder Plus aren't supported in Sitebuilder Desktop.