The Serial port on your computer uses voltages from -3 to -25V to transmit a logic "1" and a voltage from +3 to +25V to transmit a logic "0" as defined for the RS232 standard. On most PCs these signals swing from -13 to +13V.
The Merlin/Orion/Skywatcher remote head requires different voltages (called TTL levels) that are 0V (digital "0") and 5V (digital "1").
|digtial "1"||-25V to -3V||5V|
|digital "0"||+3V to +25V||0V|
So in order to connect a Merlin/Orion/Skywatcher remote head to a computer you need a voltage level converter between the two devices that not only inverts the voltages for digital "1" and "0" but also uses different voltage levels.
Unfortunately there is no off-the-shelf solution available so you need to do a bit of manual soldering
Tronisoft sold these converters for some time but now lists them all as "out of stock":
The Merlin/Orion/Skywatcher cable connector is na RJ12 (phone connector) with 4 wires: GND, +12V, RX, TX
Details about the cable (french): http://www.autopano.net/wiki-fr/action/view/T%C3%AAte_astronomique_Multi-fonctions_Merlin_Interface
Other untested options are:
There are several wireless solutions that connect computer and Merlin/Orion/Skywatcher remote head via bluetooth (like: http://www.ursaminor.hu/). These should work fine with Chronomotion as long as the bluetooth connection acts as a virtual serial interface. Chronomotion has not been officially tested with such bluetooth devices yet but we are very interested in reports of someone who tried.