The Flex theme is sold outside of the Theme Store, so adding the theme to your Shopify store requires a different approach.
Register your theme
To access the download link for Flex, register your theme purchase(s).
Open the order confirmation email to get your personalized registration link. This is preloaded with your order number and purchased themes.
Simply add your email and shop domain to the labeled fields.
Click Register when all fields are complete.
Download the zip file
Once the theme is registered, return to your inbox to open the Registration Confirmation email.
Included in this email is a unique URL for your theme files.
Open this and click the link beside Flex to download the theme.
The theme will download as a ZIP file called unzip-me-first.zip. You'll need to unzip this file, which creates a folder called Welcome to Flex.
Open the "Welcome to Flex" folder
There are 14 ZIP files in the Welcome to Flex folder.
There will be a main flex.zip file, as well as zip files named after the current 13 demo shops (ex: flex-swagger.zip).
Upload your preferred preset
Open your Shopify admin and navigate to Online Store, then Themes.
Under More themes, click the Upload theme button.
Click Choose File and select the ZIP file for the preset, then Upload.