- From Setup, enter Installed Packages in the Quick Find box, then select "Installed Packages" to find the installed package that has available licenses.

- Click the Manage Licenses link next to the Mass File Download name.

- Click "Add Users."

- To select individual users, use the checkboxes. Selected users are listed in the Selected list. When the list includes all the users to whom you want to assign licenses, click Add.
- To select all users for the current view, click Add All Users, then click OK.

PS: View Mass File Download, Salesforce file download app on Salesforce AppExchange: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EcsAOUAZ