When connecting to this data source, all you need to do is to your Google user and select your desired report.
Please note that Google Merchant Center is a premium connector which means that you need to contact us in order to get access.
If you you are curious to what data you can import, see the article What data can I get from Google Merchant Center? If you have any questions please contact firstname.lastname@example.org or write to us in the in-app chat. We're happy to help :)