Got it working now! It's a weird thing that it works, but it does... I only hope short-circuiting the CV and Gate doesn't damage the Bass Station. I'll post some track with the sx-150 sooner or later...
thanks for the help!

sorry, i didn't know much bout the sx-150. lol. finally looked at the schematic.

I had read the text... but hadn't understood that sentence until now. he means the part of the wire connected to the tips of the plug, so it's not the ground, it's the phase (or how it is called in English).

Thanks for the reply!
Which synth do you mean, the Bass Station or the SX-150? The Bass Station has two separate outputs for CV and Gate, with the usual 1/4 Jack connectors. The SX-150 (the small DIY one) has some sort of CV input with a 1/8 mono jack... I say some sort of CV because it's not using any standard, I guess it's meant to be used with the Gakken Theremin kit.
Anyway, in the video it looks like it's the ground he's winding up on the stylus, but I'm not sure, because it could be a second cable (it looks like there's some brown plastic around it...

Here's what the dude that made that video had to say:

"The Gakken SX-150 stylus gets the CV and GATE information from the ribbon-like slider pad.
I just connected the Novation BassStation Rack CV and GATE out to the stylus and voilà! - have my MIDI controlled Gakken.
The cheapest way to do this is to cut a patch cable in half, took the plastic bits off the wire coming from the tips of the jacks, roll them together and stick the Gakken's stylus into.
You may notice that it only works if the BassStation's CV type set to '00' (Roland/Sequential/Arp) - because of the voltage and plug the Gakken SX's out to a mixer rather than using the built in speaker - this is needed as the ground.

I believe this works with other MIDI-to-CV converters too."

CV and Gate use 2 wires per.. so a normal guitar cable will work for each signal.

He simply cut 1 cable in half. wired the two ground (shields) together
ran that to ground on the DIY synth.. then plugged the CV wire into the CV input
adn the Gate wire into the Gate input.

**edit**

I dont know that synth at all but it would appear that to save panel space they used an 1/8" stereo input for the CV/Gate signal (since typically when you're using CV you're using gate at the same time). So he's just splitting the 1/8" stereo cable to two different cables to connect to the Novation bassstation.

http://en.wikipedia.org/wiki/CV/Gate

I just bough a Novation Bass Station, which is really a nice little analogue synth, and I got it for a great price...
Anyway, I found this video here, showing how to control a Gakken sx-150 through the CV Gate of the Bass Station: