I purchased a security camera directing to set channel selector to av channel??

??

Thanks

Do you mean AV as in Composite, or TV Channels that have Numbers?

Your question is quite vague, but there aren't any cheap HDTV modulators out there and most camera's are NTSC (The old analog modulation), Do if trying to view under chan 3 or4, it likely has to ba analog channel 3 or 4 and not digital channel 3 or 4.

AV channel may mean audio-video channel.

You have to tell us more about the cables used and what the manual tells you about the output format of the camera.