Finally

It was much more easy than I thought...
following LeBer's advice, I went to the wikibooks and found that I better use the tool /Developer/Tools/WSMakeStubs.
It creates two objects (in my case Account.[hm] and WSGeneratedObject.[hm]) which I simply imported in a new project and, via some controller, they can invoke the service... I had to write the following two lines of code to see what was going on:

@implementation CPO
- (IBAction)servicio:(id)sender
{
 id valor = [AccManagerService newAcc:[nombre stringValue]
                               in_who:[quien stringValue]];
 NSRunAlertPanel(@"Servicio",
                 @"El valor es = %@",
                 nil,nil,nil,valor);
}
@end

Nice and beauty...