First, you should have downloaded the most recent version of the MARE2DEM MATLAB routines from the download page. Unzip that folder if you haven't already and place it in a safe place where you won't risk deleting it by accident.
Now you have to let MATLAB know where these files are located. This is done by adding them to the MATLAB search path. So start up MATLAB and find the "Set Path" menu item (or preferences item, depending on your version of MATLAB). Select "Set Path" and then hit the "Add with Subfolders" button. Select the MARE2DEM MATLAB folder you downloaded and then you will have 5 folders added to the MATLAB search path, similar to what is shown in the figure below. Hit the "Save" button so that MATLAB will store the paths for future sessions.