MATLAB needs to know the folder where the stamps scripts are located. You can do this in the GUI (by adding this directory to the list of folders by clicking on “Set Path”.
Another option is to add this folder to the line where the MATLABPATH is defined in the StaMPS_CONFIG file
(line 45 here: https://github.com/dbekaert/StaMPS/blob/master/StaMPS_CONFIG.bash)