Hello miriggall!
It sounds like you need to configure an existing DHCP scope or create a new scope. Essentially the DCHP scope protects for the IP address range that is supplied to the workstations in question.
Please refer to the following TechNet resources to assist you at:
http://technet.microsoft.com/en-us/library/cc787068%28WS.10%29.aspx/cheers
