System Manager 2.2RC1 replaced System Manager 2.1 on the now site. System Manager 2.2RC1 is an update to System Manager 2.1 and has bug fixes and enhancements over v2.1
System Manager 2.1 has been archived. You can still download it from the downloads page. You need to go to the end of the page and select "OnCommand System Manager" from 'To access specific version' drop down.
It is recommended to use the latest System Manager version ( in this case v2.2RC1 ) as it will have improvements over the previous version.