i mean i want to use the captive portal with local radius can i do that is pfsense ? i dont need any accounting things i need only to get username and the amount of MB of internet usage he used.
On your first question answer is - Yes, you can use CP with local radius.
Your second statement/question is contradictory - amount of traffic IS accounting.
With local radius you can do some "primitive" accounting in a way that CP is generating start/stop messages to radius which write those in /var/log/radacct/users_IP address directory (like this one /var/log/radacct/192.168.151.252). Inside you will find file for each date (detail-20100608) in format detail-date_stamp.
I have attached sample file which I have created for you. In those files you will find data how much of traffic is generated by user from this IP for each individual session. It is up to you then to transfer those files to your accounting computer and do something meaningful with that data.
I hope that I have provided you with info needed.