The 'libraries' folder in the sketchbook folder is for contributed libraries, not core libraries.
Try going to Sketch > Import Library and see if "Serial" shows up...

By any chance, are you working in Android mode? Because that doesn't support serial...

I don't know what is Processor. Is that a variant of Processing?

The serial package is indeed bundled with Processing and should be available out of the box. As calsign said, perhaps you try to use it in Android mode.
Also it can be useful to indicate which version of Processing (or Processor?) you have downloaded.

Can you show your code and the tutorial in question?

Wow, calsign, you have good eyes! (And I suppose I only glanced at the given line, looking familiar. I usually check for typos or wrong case.)

matelot2, most of the errors come from Java, although this one is generated by Processing itself. I suppose it catches all kind of errors happening in these imports and outputs a message that tries to be helpful for newbies in most of the common cases... (correct syntaxes, wrong setup).

If I reproduce your error in Eclipse (a Java IDE), I get the following errors:
Multiple markers at this line
    - The import processing.serial cannot be resolved
    - Syntax error on token ",", . expected
which at least point to the right mistake... I suppose Processing caught only the first error.