@JustinVH I just was puzzling with the rotation and found it. The insert point and the snap is working fine know.
The only thing I don't find is how you can connect the arc to the top or down on an other truss.
Like if you connect a c003 corner of prolate to a truss, you can choose the orientation of the corner leg. left, right, down and up. Is this also possible with a single circle arc?