You must select three elements when you add a Motion-Dimension FB for a Rocker. They are:
•...followed by two Line sketch-elements.
However, you may find you cannot select one of the Lines to complete the command.
This can happen when:
•there is already a Motion-Dimension FB at the Pin-Joint
•the Point at the Pin-Joint is not a child to the Line that radiate from the Pin-Joint.
•the Pin-Joint that you select is not a Pin-Joint between the two Lines you want to select.
Read this topic: Add Motion-Dimension.