Press the Connect button on the module page in RetailCRM marketplace (Settings > Integration > Marketplace).
Paste the API key you've just created to the API Key field on the opened module configuration page. Leave your contact e-mail to receive module updates notifications. Press the ✔ button. If the key is incorrect, or some necessary permissions are missing you will see an error message:
Click the Connect button in the bottom left corner of the page. On successfull connecting you will see the + Handler button in the left-side menu.
If you are sure that you specified a correct API key, but you still see the error message "Check url and API key", make sure you've created at least one store. RetailCRM may return an API error if there are no shops in the CRM account.