Sorry to be slow in responding, but I cannot replicate this problem. SU's Install Extension option works as expected on my Mac which is running the latest version of Mountain Lion.
As I mentioned previously, the plugin is not designed to accept two points via the VCB.
Perhaps these two videos would help you understand how the interactive versions are designed to work: http://www.youtube.com/watch?v=M4QUO0PaI_E Youtube Video
Dear Max,
Clever and instructive explanation.
And certainly a very useful tool. (i have fought some time ago with that spiral shape and haven't found in this time such a tool
http://i274.photobucket.com/albums/jj245/Spendauballet/SketchUp/horseshoestair_Final5.jpg
)