Is BGP going to your ISP or what? To advertise the class C all you have to do is type in "network XXX.XXX.XXX.0". Keep in mind that will only advertise that network, it won't configure it to send and receive updates.
Are you able to use the entire Class C or is it subnetted? If it is subnetted you need to add a wildcard statement to the end of the network statement.