Please watch this video demonstrating the process:
Adding Modifiers to Menu Groups
From within the design editor, select a Menu Group. The Text tab will appear on the left. Select Style.
Scroll down on the Style tab to Modifiers. Select the toggle to turn Modifiers on.
Once Modifiers have been toggled on, they will be accessible within the text editing box.
Select +Add Modifier Group.
Enter text for Modifier Group 1, Modifier 1, and Price 1.
Select +Modifier to add additional Modifiers.
Style Modifiers with the options on the left.
If Modifier Prices are In-line, they can have Dividers, but Dividers will not be available if Modifier Prices are Stacked.