That button worked in previous sketchup/toolbar editor versions.
I suppose the code of that button is similar to that of the other buttos (which are ALL available in TE)
Installing the latest version of fredo splines fixed it.
The relase note claims that this build fixes problems with toolbars resolution. That’s it. 
Thank for the help. Looking forward to toolbar editor improvement and fixes. (trimble should buy this and incorporate in the core program IMHO, and a bunch of other tools too)
edit:
i noticed an odd fact: the bezier classic button works fine as long TT selection toys is uninstalled, but when i install such extension, the “select edge loops” button from selection toys overrides the bezier curve button.
Sort of “button conflict”. What a mess. 
edit - 2 @DanRathbun
I looked at some of your post to send keystroke by ruby code and tryied to use it as a workaround to send a shortcut creating a custom ruby button from the same custom toolbar plugins, but looks like i’m too noob and dumb to make it work.
What am I doing wrong here?
edit - 3
this works!!! hell yeah! looks like it was only a sintax error for “alt” key
