To take advantage of the offer, visit this page to sign in to OneDrive and then verify your Dropbox account. It appears to work for users across the globe.
If you’re invested in Microsoft’s suite of apps and services, this is certainly worth grabbing. OneDrive already offers 15GB of space for free, over Dropbox’s 2GB. Personally, I’m happy to take advantage of the offer and stash a bunch of RAW photos I don’t need immediately or often.