Okay, DSL user here so bear with me, never setup cable before.
Is it a USB cable modem or one that plugs into the enthernet jack? If it plugs into ethernet then it still shouldn't be a problem, I'd imaging there would be some sort of config web page you'd browse to. My router is http://192.168.1.254.
If this is the case and it's not working it's linux chucking a fit at your ethernet card, nothing to do with the modem.
If it's a USB modem then you need drivers for it, if you're lucky you'll get specific ones, if not you'll have to search for some generic drivers and hope for the best. I've played with USB DSL modem drivers before and it's not fun in any way, shape or description.