No, you won't be able to use the "LinkSys Phoneline" technology with your Macintosh. You are potentially loosing out on the full performance of your cable modem through-put by using this "PhoneLine" connection.
I imagine your cable modem comes in one room and you have the computer in another and you are using your in-house phone line as the transport between the two, correct?
One way, not my recommendation, but possible would be to run a cross-over ethernet cable between the Windows and Macintosh, and turn on Internet sharing on the Windows box. If you got it all setup right, this in theory would work using your phoneline technology.
As the previous poster said... spend the cash and go with wireless. 802.11g is going to be the ideal solution without running cat5 ethernet cable in your house.
This "PhoneLine" connection is nothing really standard and is more of a "it's possible, but not recommended" technologies that are available.