Currently, you cannot backup Guest machines running inside of VMWare Player.
The VMWare agent documentation says:
“Backup of Virtual Machines available under VMWare Server/VCenter is supported using VMWare Plugin. The plugin uses VMWare API's to backup the selected Guest OSes and then upload it to the backup server. Extend Backup provides an agentless backup of virtual machines. The client can be installed in any machine to backup the Virtual Machines”
The documentation also says:
“VMWare workstation is not supported.”
The best option for backing up guest machines inside of VMWare Player would be to install the appropriate Backup client software (available from http://www.extendgroup.net/downloads.html), directly into the guest machine, and configure backup of the required data as normal from there.