Have you used your right index finger on the Karimba yet? That note symbol below that looks like an "I" indicates that you play with your right index finger. You can pluck up, or push down with the index finger's nail.

Hear what this riff sounds like

The right thumb and finger make a chord which is played three times, then a different chord which is played three times, and repeat. That chord progression forms the structure of the riff. The left thumb plays a simple melody, gently weaving in and out of the right hand's simple chord pattern.

Try this: keep the right hand steady, and let the left thumb off the hook: let it be free to do anything!