Do you have Dropbox on your computer? If not, you might need to install Dropbox on your computer and then Log from your computer. I don't know exactly how it works. All I know is when I logged to Dropbox I had 50g available.
Edit: Apparently you need to register to Dropbox from the initial setup from when your phone was new. So you might need to make a factory reset. It might also not work with custom ROM.
I tried logging in to my old Dropbox account but haven't used it for ages so had to change password. I skipped the login at the initial setup, changed password on the Dropbox website and then logged into the app with my existing account. I then got an email a couple of seconds later saying I now have 50GB.