Possible reason ONE: You are using the Office 2007 version.
Office 2007 doesn't allow you to log in and only allows you to use the offline features. You are recommended to upgrade your Office version.
Possible reason TWO: You have installed too many versions, causing the profile confusion.
Open the following location: %LOCALAPPDATA%\Microsoft_Corporation in the address bar of Explorer
Find all the files, delete them, restart PowerPoint and enable iSlide.