6months ago a site with it's own domain joined my network, now the domain is up for re-newal and the owner doesn't want to re-new it but says he will help me transfer it to to my registar, which is dotster.

How do I go about doing this, I've e-mailed them (Namezero) but got no reply yet. Any advice appriciated.

This is Dotster's job. You initiate the transfer from the gaining registrar, and assuming your client has the authority to authorize the transfer through NameZero, the transfer will go through.

Start with tech support at Dotster.

-t

Will this cost me anything?

Transfers between Registrars result in one year being added to the end of the current domain's registration period, so generally you will pay at least your new Registrar's annual domain renewal fee. This additional year is a Registry requirement.

Some registrars will charge for failed attempts at transfering domains, some will charge for transferring away from them, etc.

It all depends on the individual business models of the Registrars involved in the deal.

-t