Avira Free Antivirus for Mac fully supports MAC OS X 10.9 (Mavericks) since version 2.0.1 (Update 1). If you are already using version 2.0 of Avira Free Antivirus for Mac, just make sure that you perform updates regularly to receive the latest updates.
If you are still using version 1.0 of Avira Free Antivirus for Mac please update to the latest version. All required steps are described here.
If you are not sure which version of Avira Mac Security you are using please perform the following steps to retrieve your product version: