DHCP permissions across domains

We have two servers on a domain running DHCP wins etc with a domain sitting below with the company servers running the applications. Both domains have AD. I have been asked to install RIS to roll out our desktops. I have installed RIS but cannot authorise DHCP with the following message. ‘DHCP authorization of the remote installation failed. This may occur if active directory is unavailable or if the logged on user has insufficient permissions to complete this task’ I suspect as stated this is a permissions issue but I am not sure how to give rights to DHCP accros the domain. All and any help would be appreciated. Thanks