authentication error w/ 'set cookie'

[Mark]

Hi,
I'm still having trouble with 'set cookie', I'm hoping someone can 
help me debug this. I can successfully register a domain, but then 
when I try to get the cookie like this:

$cmd = array(
   "protocol" => "XCP",
        "action" => "set",
        "object" => "cookie",
        "attributes" => array(
        "reseller_ip" => "66.250.26.107", 
                "domain" => "educationplanetxyz.com",
                "reg_username" => "user",
                "reg_password" => "pass"
        )
);

I get an authentication error. any advice is appreciated.
thanks,
- Mark