Making a ragdoll requires you to use the hierarchy tab in the command panel, which may be a new area for less experienced modelers. Watch the video here to get a basic overview of how to make a ragdoll for Source.

Ragdoll Requirements

To make your ragdoll, you must create a linked hierarchy. You must also set the rotational limits of each joint. The items marked with an asterisk (*) should be skipped for the root (parent) of the model.

1. Click the Pick and Link tool (usually at the top left of the 3ds Max UI) and link all children objects to their parent objects.
2. Select each object and click the hierarchy tab in the command panel.*
3. Set the hierarchy submenu to IK.*
4. Set each axis to active and limited.*
5. For each axis that should not move, choose From 0 to 0.*
6. For each axis that should be able to move, set the desired angles.*
7. Make sure that the model is either skinned or each object has Bone On (you can get to this by clicking Animation > Bone Tools and scrolling down to the bottom of the bone tool floater).
8. In WWMT, make sure that you have $jointconstrain checked in the Collision Hull rollout.
9. Export.