Yes, I was indeed quiet in here until last night, when the forum got suddenly flooded with spam. So, I started to clean up the forum, and while doing it I realized there was actually some activity, and some open questions to be answered. So here goes:
Grounding the Kastle with 4060MK2's SLEEVE is a good starting point. You can do this with a juper wire or by connecting the two machines to the same mixer. If you have not grounded the two together, patching just one wire from a box to another will not be able to transfer a signal. Having said that you can usually send send signals back and forth by connecting two wires between the machines, without grounding them. The last mention is experimental though.
I quickly read what Kastle manual says about connecting external stuff, and it says the synth responds to signals between 0-5 volts, and that the signals will be rectified. I would still say, that the device can stand signals outside that range, since most modular synths produce larger swings. But that's just my impression.
If you want to be absolutely sure that your 4060MK2 will not produce larger signals than 0-5 volts, you can use a 5 volt wall wart. 4060MK2 will work with DC power from 3 to 15 volts, and will produce signals according to the operating voltage. Also make sure that the jumper in in the leftmost position (SLEEVE to minus). This will make the device unipolar.
I have used both 4060 versions with a bunch of different equipment and to my experience the current it can provide is so little that it's very unlikely to damage stuff. One simple way of limiting the current is using a resistor to patch the signal, instead of a jumper. You can start with, say a 100k resistor and try smaller values if there's not enough effect.
You can also lower the output voltage by forming a voltage divider with two resistors.
