If you are unable or having difficulty downloading a certificate for which you were approved, here are some steps you may to take to resolve this.
First, check that the certificate has been approved and is not still in pending status. You should be able to see an "Approved" status as well as a "Download Certificate" or "Re-download Certificate" button on your record in Match Activity:

If the Download Certificate button is not available, please check the "Status" to ensure the certificate is ready--in some instances, donor companies choose to customize certificates themselves, and these will not be available immediately, but rather when the company uploads it themselves.
Where the certificate is downloaded on your computer will depend on your internet browser's settings. Depending on your settings, common places are either the desktop, a "Downloads" folder, or another designated place, as determined in your "Preferences" settings.