Consider this an option of last resort - students should upload their own work and save you the hassle. But there are plenty of times when you'll have to be the one doing the uploading.
The process is super-simple. Log in and click on your assignment. Go to the "upload" tab.
Look for the box that reads "Instructor upload to:"
Select the class section that the essays belong to, and hit "upload essays". You'll be taken to a file selection screen where you can upload all of that section's essays at once.
There's also a tutorial video for this process: http://youtu.be/G3u2xer2oTE