By default, QuickBooks Online has class tracking disabled. If you want to track classes using SellerZen, you'll have to first enable classes in your account settings.
Go to the Gear Icon and then click on Company Settings.
The option to enable class tracking should be under:
Company Tab > Class Tracking
Advanced Tab > Class Tracking
Click on the Pencil icon to edit and turn on Class Tracking.
You'll want to make sure you track classes for each row in transaction instead of the whole transaction if you want to track individual line items (income and expenses).
How to view and add classes
Click on the Gear Icon to view All Lists. Then click on Classes to view and add new classes.
You can also make the new class a sub-class. For instance, you may have Kitchen, and underneath, sub-classes like Appliances, Utensils, and Cleaning.