The Flex Theme Horizontal Tabs are implemented as a preset on the Visual Composer Tabs Element.
Note: There are two presets, one for Desktop and one for Mobile. If you wish to have the first tab expanded on desktop and all tabs collapsed on mobile, you must place the tab element in twice in two separate columns, one set with the Desktop Preset and one set with the Mobile Preset, and set the mobile and desktop visibility. You can use the Visual Composer Copy Option to create the second instance and then change the Preset.
Add the Tab Element to your row.
Choose the Arrow Next to the inserted Tabs to Reveal the Edit Tabs Option
Click the Edit (Pencil) Link to open the Tab Edit Modal and then the Gear icon to reveal the presets. Choose GSU Tabs or Mobile Tabs and Save changes.