Go to FTP Manager > FTP Accounts.
This page should contain the info and options you'll need to add an account. Think you may need to create the directory for the FTP account before you add the user. If you have any more queries post back.
EDIT: Sorry, just re-read your initial post and realised that you may need to limit their ability to write other files in that same directory. If that is the case, I don't know. If you just meant that you don't want the user to be able to write files in other directories then follow the guidance on that page I mentioned above.
In other words, cPanel lets you limit an FTP account with read and write priviledges to a certain directory, but as far as I know it doesn't let you assign 'upload only' priviledges.