How to read a treadmill ear clip pulse sensor
I'm pulling off of their wiki, but I don't think you need an amplifier.  http://www.seeedstudio.com/wiki/Grove_-_Heart_rate_ear_clip_kit
If you check out their measured signal, it's a very clean 5 volt pulse.  Sensors are never that nice.  The white box inline with the signal is likely cleaning up the signal and doing some thresholding.  This may be either good or bad for you, depending on how much control you need.

Their example code will be a nice starting point for actually measuring heart rate, although it could definitely be improved significantly.

As for the pin-out, we're left guessing.  If we look at the base shield, we can see that it's labelled, from the top-most pin, "D1, D2, V, G".  Comparing that to the ear-clip's connector, that tells us that yellow should be our data pin, red is 5 volts, and black is ground.

Now, if you're looking at using just the clip, without the additional signal processing module, things get a little saucier.  Now you probably have to do some reverse engineering.  If you have the signal processing box available, plug a 3.5 mm plug in, and measure for a voltage.  It may or may not be 5 volts.  You may be able to assume that ring2 is ground (maybe).  Tip might be signal, but who knows.

If you're really lucky, you may be able to figure this out without even the signal processing box.  Chances are, the ear clip is just an opto-coupler.  One side will be a transmitter, maybe an infrared LED, and the other will be a receiver, maybe a photodiode.  Your pulse will change the amount of light allowed to pass through your ear, giving you a nice, noisy, analog signal.  You could test this by grounding what you hope is ground, and putting a current limited voltage through something like a 100K resistor into another input.  Keep the clip in darkness so the photodiode won't allow any current to flow.  Slowly increase the voltage until three volts or so, at which point the IR LED should have turned on.  Measure from the resistor-clip node to ground.  If this is less than 3 volts, that means current is flowing, which means you've found the LED and ground.  This leaves the remaining pin to be the photodiode (i.e; your signal).

Hopefully that will get you started.