From TDN

Brief explanation by Rick J. Kelley AKA Rat:

A MountPoint is for the weapons placement or attachment to a player, or a vehicle. This is not the same as a Mount node on player or vehicle.

The MountPoint node is needed on the weapon and should usually be placed where the player's right hand would be. If there is no mountpoint and you have exported it to a .dts file, then the engine will default to 0,0,0 as the origin for the dts file to mount to the appropiate "mount" node on the object that you wish to have it mounted to. To create a MountPoint in 3D Studio MAX; you would go to the create tab and choose "Helpers," then click on "dummy" and create a dummy in your viewport. Rename this dummy to "MountPoint."

This mountpoint node needs to be linked to the body of the weapon in the "Schematic" view of MAX.

Alternately; to create a mountpoint node in Milkshape 3D, you would simply create a joint by going to the Model tab of milkshape 3D. It should be moved to where the players hand would match up.

In Houdini, this done by using a Null node. Again, place the node where the players hand would match up.