Once you set up your DMARC record in your DNS settings, you should give this a little time to take action. You should then see the DMARC icon on your control panel turn orange to indicate that you have now set up DMARC to report only mode. It will take around 24 hours to receive your first DMARC reports.
You can also check you have inserted the correct DMARC record by going to the analyzer and entering your domain. Once it has finished loading you can scroll down to check your current DMARC record and make sure it matches the record OnDMARC gave to you.