Generally using the synthesizer isn't that difficult. It is just a shitty little monosynth after all… or so you might think! The TB-303 and by extension, the x0xb0x, have some interesting little quirks that affect what kind of sound that you can get out of it.
The VCO is primarily controlled via the sequencer. Howver, you can perform a cv_in modification for external control. The 2 controls of the VCO are the note, and the slide from the sequencer, The Tune knob, and the Tune and Width trim pots inside.
You can use the Tune knob to adjust the tuning of your x0xb0x. Be aware that this will only adjust the tuning of the internal VCO, it will not have any affect on the CV output.
You can also further adjust the tuning of your x0xb0x by opening it up, and ajusting the trim potentiometers TM4 and TM5. TM4 is the fine-tuning of the oscillator, and TM5 is the 'width', which determines the octave width for the oscillator. For more information on calibrating your vco, see VcoTesting.