Oh there was one thing that has changed since that tut was made. After attaching the link to the motion path and creating the motion path snapshot...

1. Select the link and then open the window/animation editors/graph editor window
2. section the motion curve for the link
3. and then at the top select the tangents drop down and choose "linear"

In older versions of maya there was "ease in" and "ease out" as options but I think they have been removed so just change the start and end tangents to linear and the link will move a fixed distance for each step.