Take a look at this thread on eHam:http://www.eham.net/forums/Digital/1675
You need to
a) feed the output of the Kenwood receiver into the sound card "mic" input,
b) feed the sound card "line" or "headphone" output into the Kenwood "mic" input,
c) arrange for the Kenwood to switch into "Transmit" mode when the sound card is producing sound.
(c) can be handled by using VOX, if the Kenwood TS-50 has VOX.
If it doesn't have VOX, you can switch it manually, or build or buy an interface that lets your computer control the TS-50's "push-to-talk" switch.