It is best to experiment close to your computer with the USB cable still connected to the Circuit Playground Express. If you are using Mu, you can click the REPL button and some debug information will show: stating R.O.B. has started and groups of 3 numbers read from the Circuit Playground Express accelerometer (x, y, and z values respectively).
Hold your Circuit Playground Express about 12 to 18 inches from R.O.B.'s head with the IR LED not blocked by your finger and aimed at R.O.B.'s left eye (on your right). Don't shake the Circuit Playground Express yet, hold it steady.
Now press the A button with a finger. R.O.B.'s arms should open wide (if closed). Press button B on the front of the Express and his arms should open. Hey, my robot works!!
The other controls are a bit trickier - some practice will help. If you turn the Circuit Playground Express board clockwise, R.O.B. should twist one way. Turn it counterclockwise and he should move the opposite way. It may take some practice as the the board is reading the changes in acceleration relative to the pull of the earth. Short, quick movements work best while keeping the IR LED aimed at R.O.B.'s head. Tricky but doable.
The final movement involves a quick up movement to have R.O.B. move his torso up. The silver USB connector should be moved up in a short quick movement (without twisting). R.O.B. should move the whole body/arm assembly up. A quick downward movement towards the ground should move the torso down. Again, keep the IR LED pointed at R.O.B.'s left eye. It may take some practice.
Once you have the basics down, you can unplug the USB cable and use the battery pack to make a portable solution. Go have fun with your robot!
If you find the movements unsuited for continued use, there are options. You can change the code to increase or decrease the sensitivity of the movements. Use the values output by the Circuit Playground Express to the REPL to guide you on values which may work. Remember that the earth pulls down (in the y direction holding the CPX at R.O.B.) with a value of -9.8 meters/second. Movements add or subtract from that y value.
You can add additional push buttons to the Circuit Playground Express pads as a more advanced project. Pads A1 through A7 are capacitive touch switches which can be programmed to activate functions. This is used in the Youtube video on the Overview page.
You can read other buttons to trigger movements. Push buttons should be momentary closed. In that case you might consider some sort of holder/case so you are not juggling your fingers.