Use an RCA-to-1/8" stereo Y-cable adapter, & plug from the Line Outs of the 414mkII to the Line In on your typical soundcard. This should give you a stereo signal from the Tascam to the 'puter.
The cable you need has two RCA connectors on one end, and one 1/8" stereo plug on the other end. It's an item that's pretty easy to find. A single speaker cable won't cut it.