When first setting up your Magento store with our Extension, you'll be required to configure your product attribute mappings which will then be found in Feed Tool > Magento Mappings:
From this section you can select the attribute from your store to be matched to the appropriate / required attributes outlined by channels like Google.
Note that unmapped fields will present a dropdown for 'Magento Field' from which you will select the appropriate value imported from your Magento store to be mapped:
Click the dropdown and scroll through the list of available values to set:
Scroll to the bottom of the page and click Save to complete you mappings:
Note that you'll have to send your channel feeds after making any adjustments to have them reflected in platforms like Google Merchant Center.