mx_hiddenmanager = SketchupExtension.new "Hidden Manager", File.join ("mx_hidemanager","mx_hidemanager.rbs")
This can confuse Ruby - you have space after the method name and the brackets. ( File.join ) On top of that you have that as one of the argument of another method call ( SketchupExtension.new ) where you don't use brackets.
To avoid potential errors, always use brackets. But most important of all, never leave a space between the method name and the brackets.
It's a bit odd that a SU7 user should report this error, because SU8's ruby is more strict. Regardless, this should work:
mx_hiddenmanager = SketchupExtension.new( "Hidden Manager", File.join("mx_hidemanager","mx_hidemanager.rbs") )
i got this error:
No such file to load -- mx_hidemanager/mx_hidemanager.rbsError Loading File mx_hidemanager_loader.rb
No such file to load -- mx_hidemanager/mx_hidemanager.rbs