Mass File Download provides a large number of features that can be customized according to the users' needs but if you are in a trial period, you will not be able to enjoy the unlimited downloading limit.
For a Trial Period user, you can download up to 25 MB of total files. With that limitation, the user won't be able to select the unlimited download feature.
To increase your download limit or do an unlimited download, you have to Subscribe to the Mass File Download.