il look again. I had no luck the other day
*link doesn't work*
Try this link or just google the company name: healtech-electronics.com
You could just try to randomly plug-in the indicator into diagnostic port, but use caution: I would assume two wires are +/- DC power (red/black), and one is signal (white wire?). Alternatively (and probably safer) you can connect red/black wires to external 12 VDC power (e.g. jump directly to battery, or use an external jumper pack to power it by clamping red and black connectors to the corresponding jumper wires), and try poking the white wire into diagnostic plug of your C50. If I'm not mistaken, it is located on the right side under the plastic cover (it usually has C50 badge glued on). Loosen one bolt facing towards the top, and carefully pull the cover off. There a few plugs, and smaller one is for dealer diagnostic. If the gear indicator you purchased from eBay has the correct size/shape connector plastic, it should match the bike's plug side. Plug in the gear indicator, start the bike (might not work without the engine running) and start switching gears. Also, you might not get a reading if rear tire is not turning (depending on how the gear position sensor works on C50 - I don't know that part).
When you find the correct signal wire, check with DC volt meter (aka multi-meter), which two wires feed power to the dealer diagnostic port (should be stable somewhere around 12-14 Volt DC), and wire it up using the plug.
Attached, is a picture of dealer diagnostic plug and wiring diagram for 2006 C50: top right corner of the diagram (next to ECM box) shows two plugs with nothing connected to them. If you match the colors of the wires going to the dealer diagnostic port on your bike, you will be able to trace which wires feed DC power, and which are signal. Considering dealer diagnostic plug has only 6 ports, I would consider checking the wiring diagram for the plug on the right side of the screenshot. Should be easy enough, if the indicator you've purchased works.