Plug the Macintosh into the router with an ethernet cable, then, depending on how the router is configured (DHCP, static DHCP, static IPs, etc.), configure the Macintosh Network settings accordingly.
In a typical setup, you would simply choose "DCHP" in the Network pane of the System Preferences on the Mac, then plug it into the router with an ethernet cable and away you go.
Beyond this, it's pretty tough to troubleshoot your problem with what information you've provided. What kind of Macintosh do you have, and what version of OS X is it running? How is the Network pane of the System Preferences configured? What model of router do you have, and how is it configured to hand out IP addresses? Is your ADSL service PPPoE? How does the PC work on that router and how are the PC's network settings configured?