Hi Sh:
This is going to be hard to diagnose at a distance, because different brands of computers are built differently. I have 3 sockets on mine -- with little pictures instead of labels -- but the pictures lead me to believe one is "line out", one is "line in", and one is "microphone in". It's entirely possible you have only a LINE OUT and a MICROPHONE IN. Sometimes a microphone plug is a little shorter than a LINE IN plug, so (theoretically) a microphone plug will fit into a LINE IN socket and make a proper connection.
There are also connecting cords, with male plugs on both ends, made for feeding line-level signals into microphone-only inputs. These special cords have "pads" (sound level attenuators) built into them, so the microphone input does not get overloaded (distorted) by a large incoming signal. I bought my adapter cord at RADIO SHACK.
If you have only a MICROPHONE input, then if you plug in a line-level sound source (using a padded connecting cord), the sound will appear at the WAVEPAD input as MICROPHONE. But try all the settings -- the sound should be there somewhere.
Sorry I can't offer better advice (without actually looking at the back of your computer case).
Jerry