A Y-cable will convert a stereo source into a mono source (2 inputs-->1 input). You can also connect the Left output of the Triton to the input on your computer. You will not get a stereo sound, but you can double and pan your tracks in your sequencer to spread the sound across the stereo spectrum.
Are you using the built in soundcard on your computer? If you are, the inputs are probably 1/8", in which case you'll need an adapter for the 1/4" cables. If you're using an external soundcard/interface (with at least two inputs), you can connect the Left and Right outputs of the Triton to the 2 inputs on the soundcard, and achieve a stereo sound that way.
Unless you're going to be working with sounds that pan a lot, stereo v. mono is not a big deal.