To get a motion on your biped you have to create a script (dont be afraind it is not maxscript hehe) If you look in the properties panel of max you see a button that is called Create script. click the button and then click on on the WALK button in your motion flow graph. when you do this you will see the blue bar turns red, now click again on the WALK. you will see a arrow, and you can see the script is adjusted.