I forgot...

Your ins and outs on your sound card generally are 3.5 mm stereo Klincke connectors (I think it's called 1/8" stereo plug in USA?).

I guess your synth have two 6.3 mm mono Klincke connectors (two 1/4" mono plugs). A normal adaptor should fix this problem.

You can use normal audio cables for connecting your devices but don't use longer cables than about 10 meters because of the fact that you'll increase the cable's impedance by increasing the length. This will lead to a higher noise and a lower signal power.