Make sure the required files and libraries are installed or have met the minimum requirements (should have green background). See screenshot:
If not, there should be a button that you can click to install these files or libraries (except if they’re server software requirements, e.g. PHP).
Go to RETS tab, and enter you RETS credentials and click Verify button.
If the credentials are correct, a new section, the RETS Metadata section, should be visible below the credentials form.
If the plugin have default settings for your RETS server, it will be loaded automatically. You can adjust the settings in this form for both the initial and sync replication process. For more info: check RETS Metadata Form docs.
Once done, click Save button to save your settings.
Go back to Overview tab and click the button to install the default templates.