Where are the files newext_tags.rb and newext.rb located?
When you declare methods in newext.rb (which should be located
somewhere in \lib) you should declare them as class methods (def
self.my_method) to be able to call them like this with
Newext.my_method.
You don’t need to use the rake update command, it just copies the
files from your extension’s \public folder to the Radiant
application’s \public folder, which is not what you want at this
point. Restarting the web server is necessary, though.