Q. Where do I set the crossover control on my subwoofer's back panel?
A. If you are using a Dolby Digital/dts A/V surround receiver with its own Subwoofer Output jack, and a single coaxial cable to your sub, then the subwoofer's internal crossover becomes unnecessary and you should turn the control to its highest setting (150 Hz) to effectively remove it from the circuit. Some brands of subwoofers may have a "Bypass" switch, which does the same thing. By doing this, you are avoiding "cascading crossovers," or using two crossovers in a row, which may cause losses or gaps in the bass response. The A/V receiver performs all the "bass management" and routes the appropriate frequencies to your subwoofer from the Sub Output jack, so the sub crossover becomes redundant.
If you are using your subwoofer for 2-channel stereo with an older 2-channel receiver or amplifier (non-surround sound), then you'd use the speaker-level input connectors on the subwoofer because in most cases the receiver or amp will not have a dedicated line-level subwoofer output jack. You must then set the subwoofer crossover on the sub itself to route the low frequencies to the subwoofer. Try a setting of about 80 Hz to start, depending on how large or small your main speakers are. If they are very tiny satellites, you may want to raise the crossover frequency to 100 Hz.
Once you've got your subwoofer crossover set you may want to read more tips on subwoofer setup here.