I already have basic internet wifi setup in my office. How can I do MAC Filtering so that only selected pc can connect and use the internet. Please help. Thanks.

In Wireless Tables go to security profiles TAB create new profile add name mac filter
select mode=none in Radius TAB mark/select MAC authentication and press OK
In Wireless Tables in Access List TAB add desired Mac address
and at the end on wireless interface on your wifi card (wlan1) in Wireless TAB find security profile option (it is set on default) just select your new created profile "mac filter".

On the Wireless Interface disable Default Authentication

Then add an entry in to the wireless access list for each client.

Thank you very much all of you. I will try it.