Alexa and Subdomain

how to make alexa count my subdomain?

Dear Supporters.

I am trying to build a community site with wordpress.
now i have around 300 visitor/day on my main domian.
and i have a 1300 visitor/day on my subdomain.

the problem is alexa doesn't count traffic for my subdomian.

what should i do to make alexa count on my subdomian site.
please help