The module should be unzipped and the folder should then be registered as an extra module. This can be done in using the following command (changing <path/to/octave> to the path where you have unzipped the folder):
>> mrstPath('add', 'octave', <path/to/octave>)
To do this automatically everytime MRST starts add the line above to your startup_user.m file.
The octave module can then be loaded with the command:
>> mrstModule add octave
More information about GNU Octave can be found here: