Hello Didier,
I am also trying to install this on the Mac Version of Sketchup Pro 7. The plugins folder contains files with extensions of .plugin, not .rb. I tried both using the .rb and .plugin extensions with no luck.
When typing $align_loaded in the Ruby Console, the variable is nil.
Perhaps there has been some change the to plugin format in version 7?
I am looking forward to using this tool, so your support is much appreciated!