Can someone please tell me how to activate a class?

Nice question, bcreyn.

You can turn on the class feature from your Account and Settings in QuickBooks Online (QBO). I'd be happy to walk you through how below:

1. Click the Gear icon, then select Account and Settings.
2. In the navigational bar, click Advanced.
3. Select Categories, then Check off Track classes to turn on class tracking.
4. Click Save and then Done.
   

Once done, you can start setting up classes. To do that, you can follow the steps below:

1. Click the Gear icon.
2. Choose All Lists.
3. Select Classes.
   
4. Click the New button.
5. Add the name of the class or location and click Save.
   

Learn more about class tracking through this article; Set up and work with class and location tracking.

Drop a comment below if you need more help or other questions. It's always my pleasure to assist. Have a great day ahead.