I purchased Commerce for an existing community, and downloaded the suite again, but there is no Commerce folder in the Applications folder. How can I install this?

It is called nexus. 

 

 

 

Yes, please follow the steps in the Guide that Richard linked, making sure to upload a full set of files and not just the files for the new app. This is to insure that all apps are at the same version. An upgrade may be required, so visit the upgrade URL (./admin/upgrade/) to check first. And your new app should show on the ACP -> Applications page, with a link to install it. Just click that link to proceed.