The way I do it is to select the part whose pivot point (origin) you want to change , select the view that lts you see where the center should be (front, right or top view) and zoom in as close as possible. Open the joint editor, then put the mouse over the center of the GREEN + , a circle should appear, then drag it to where it needs to go. Drag it close and zoom in more to position it accurately. Using the textured wireframe display mode lets you see the mesh and position the center more accuratley.

 

The poser units dont really matter for this.  On the joint editor panel you will see the x,y,z values for the center and you can move those dials if dragging the + isnt accurate enough.