ok, here it goes:
1. The includepath looks like this:
INCLUDEPATH += "C:\Documents and Settings\Susy\Desktop\System\libs"\
"C:\Documents and Settings\Susy\Desktop\System"
2. For example, the include statements at arlsmat.h file are:
#include "arlspen.h"

#ifndef ARLSMAT_H
#define ARLSMAT_H
#include <stddef.h>
#include "arch.h"
#include "armat.h"
#include "arhbmat.h"
#include "arerror.h"
#include "blas1c.h"
#include "superluc.h"
#include "arlspdef.h"
#include "arlutil.h"
and the error is :C:/Documents and Settings/Administrator/Desktop/System/arlsmat.h:120: error: 'n' was not declared in this scope.
When I go to the option "Follow symbol under cursor", it goes to the definition of variable 'n' in armat.h

3. adn 4. All files are in the same folder, in the folder of the project, that will be: C:\Documents and Settings\Administrator\Desktop\System

5. C:\Documents and Settings\Administrator\Desktop\System

Thanks for helping me