Kbase Article of the Week: Macintosh TV: Television Connection Tips →

Apple Support:

Basically, anything that can be attached to a regular TV can be attached to Macintosh TV. There are two ports for a video source: coaxial and RCA. Things that have an RCA plug can also be attached to the coaxial port with the proper adapter, and vice-versa. One restriction is that closed caption must come through the coaxial port.

In the TV Setup control panel, click on Channel Setup and select Cable (as opposed to Antenna for broadcast TV) as the source.