TXT 'Dropbox' to 126222 (Vodafone) or 126333 (3) and follow the instructions.
As far as I know, I think the coupons are unique, and can only be used once.
Visit http://www.dropbox.com/coupon to add your code
If you want a code and new to dropbox, and don't have a Vodafone/3 Mobile, I can help you out!
Just sign up using my http://db.tt/HIxlhfb, and I'll provide a code to you (I got a couple)! so just send me a PM after you sign up :)