Hi Margherita, Since version 9, Plotlegends is built in and no longer needs a package.
m = RandomInteger[{0, 9}, {3, 3}];
plot = MatrixPlot[m, ColorFunction -> "TemperatureMap",
ColorFunctionScaling -> True, FrameTicksStyle -> Opacity[1],
FrameTicks -> All, PlotLegends -> Automatic]
