As LegalAmerican stated, log in to Dropbox from your Note 10.1 and you will be prompted to visit the "Get Started" page. On this page will be displayed 7 tasks - 5 of which you will need to complete to receive your additional free storage.
The tasks can be completed in any order and are as follows:
1. Take the Dropbox tour
2. Install Dropbox on your computer
3. Put files in your Dropbox folder
4. Install Dropbox on other computers you use
5. Share a folder with friends of colleagues
6. Invite some friends to join Dropbox
7. Install Dropbox on your mobile device
Before purchasing my Note 10.1 (or even knowing about the "Get Started" page for that matter) I had already completed four of the seven tasks (2-4 & 7). I immediately completed the Dropbox tour (tutorial) and received my extra space. However, I'm unsure if Dropbox will recognize your tablet as a mobile device or 'other computer' since I already had it installed on my home computer, work laptop, and Galaxy Nexus. All tasks seem simple enough to ensure you get your free 50GB as advertised. After two years, if you don't want to pay to keep it, there is no obligation to do so. I doubt I'll get my Dropbox filled anywhere near that amount by then, but it's nice to know it's there in the meantime.