Here I go then,
Select your bipeds foot and go to the motion tab. Make sure parameters is highlighted (it usually is by default) and go down and open the Key Info tab.
Inside there is a little button called 'set planted key'. This will plant the selected biped limb to the current position.
Go to the first frame you want your foot to be planted on and click the key. This will create a keyframe at the selected position.
Move the timeline across to the frame you want the foot to be released from it's plant and hit the key again, this will create two keys in your timeline, between these keys is how long you want the foot to be planted for.
Two buttons across there is a 'set free key' button.
This will create a key that removes the foot from any of the constraints set before and return it to the default biped behaviour.
Your keys should look like this;
Frame 1 is the first 'planted' key
Frame 35 is the second 'planted' key
Frame 36 is the free key.
If you place a single planted key and then 20 frames down the line you place a free key, by default 3ds max will try its best to simulate the movement between the two keyframes and the foot won't be planted.
Hope I made this clear enough.
