For the script to function properly the values should be changed to a range you want to interpolate. In my example, if I want to interpolate the whole animation I have to set values to 1 and 14 accordingly:
Hi Kacper, thanks for the quick response and sorry for my delay! I am actually not even getting that dialog, when I click on file->scripts->tweener or file->scripts->tweener dialog nothing happens at all. Maybe I downloaded them incorrectly?