First, we need to create a joint connection. Right click on Connections and select Insert>Joint.
Now, use the vertex tool to select the point where the line bodies intersect "above" the flexure mount and apply it as the "Mobile" Geometry selection. Then select the face of the flexure mount near the vertex (using the surface select tool) and then apply it as the Reference Geometry selection.
Do this for the remaining flexure mounts and the center submount. A total of nine joints should be created.