are you sure that you share this folder (look at the library window)?
If not, stay in the library window and choose share another folder in the file menu and select your desired folder.
It may take a long time if you have many files in your shared folder but you will see in the left bottom of the window for example 256/2500 and this number will upgrade.
Another possibility is that you are behind a firewall which do not permit the upload from your computer.
I hope that it could help you.
Nap.