Find the path the folder that defines the class you want to use. My guess is for Tucker this will be the ttensor class (but it's been a while for me since I used this), so you would look for a folder called @ttensor.
Call addpath function in MATLAB with the path to the folder that contains the @ttensor folder. At that point, you should be able to call the constructor of this class.
Generally, it's probably better to look for some readme file in the main directory of this toolbox, which is likely to tell you all the folders you should add to the path to be able to use all the functionality of the toolbox.
