how to hook up cable with no box to maxent plasma tv?

If you TV does not have an RF coax input then it is a monitor (no tuner inside it). You will need some kind of external tuner. What you can use depends on the cable system. If it is digital a vcr will not work. An RF modulator cannot tune channels, just puts a video/audio signal on channel 3 or 4 which require a tuner (that you don't have) to use.