Network Security Settings on a CE500 Switch using CNA

I have been trying to change the network security settings on a CE500 Switch from Low to Medium and therefore enabling 'MAC address authentication on all Catalyst Express 500 switches in the community'.

However when i go to enter a MAC Address i get an ERROR: Incorrect MAC address format

I've try xx:xx:xx:xx:xx:xx and xxxxxxxxxxxx

but with no joy

Can anyone help?

Please