As of now, the app is supported in Lightning Experience only. Here's how you can access the app in the Lightning experience:
Accessing the app in Lightning Experience
- Once in Lightning Experience, click on the "App Launcher" (the nine-dot icon) usually found at the top left corner of the page.
- In the search box within the App Launcher, type "Mass File Upload" and then select "Mass File Upload" from the search results.

PS: View Mass File Upload, Salesforce file upload app, on Salesforce AppExchange:
https://appexchange.salesforce.com/appxListingDetail?listingId=82cf236b-1ee8-4ea7-aebf-fd0c89fb93bd