xmlrpc method missing

hi guys
i'm trying to get json response from external url which format is like this :

http://www.example.com/rpc?json={"method":"get-user","arguments":{"username":"param1","password":"param2"}

now  i need to call method "get-user" using : $this->xmlrpc->method
and pass arguments param1,param2 using : $this->xmlrpc->request

but i don't know how to do this  
could any one help plz