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.
Tronisoft sold these converters but now lists them all as "out of stock": http://www.tronisoft.com/cat_rs232tottl.php