Why I can't use "differ.mac"?



I'm a beginner. Now I'm learning to use maxima from 
http://maxima.sourceforge.net/docs/tutorial/en/gaertner-tutorial-revision/Contents.htm
And I face a problem at 
http://maxima.sourceforge.net/docs/tutorial/en/gaertner-tutorial-revision/Pages/DiffEq0001.htm
It is said that "to solve difference equations, you have to load the 
package differ.mac. You do this which the following command: 
batch(differ);" I followed these words, but was reported with error:

(%i30) batch(differ);Could not find `differ' using paths in 
file_search_maxima (combined values: 
[/home/pihou/.maxima/###.{mac,mc},/usr/local/share/maxima/5.10.0/share/###.{mac,mc},/usr/local/share/maxima/5.10.0/share/{affine,algebra,algebra/charsets,algebra/solver,calculus,combinatorics,contrib,contrib/boolsimp,contrib/descriptive,contrib/diffequations,contrib/diffequations/tests,contrib/distrib,contrib/ezunits,contrib/format,contrib/gentran,contrib/gentran/test,contrib/Grobner,contrib/lurkmathml,contrib/maximaMathML,contrib/mcclim,contrib/numericalio,contrib/pdiff,contrib/prim,contrib/rand,contrib/sarag,contrib/simplex,contrib/simplex/Tests,contrib/solve_rec,contrib/state,contrib/stringproc,contrib/unit,contrib/Zeilberger,diffequations,lbfgs,linearalgebra,integequations,integration,macro,matrix,misc,numeric,orthopoly,physics,simplification,sym,tensor,tensor/tests,trigonometry,utils,vector}/###.{mac,mc}] 
) -- an error.  Quitting.  To debug this try debugmode(true);

Why? Can someboy help me?