Download the attached file. You need to push Dropbox.apk and DropboxOOBE.apk into system/app and fix permission. Then you need to push com.dropboxpartner into system/framework and again fix permission.
Then, you need to modify the build.prop using a root explorer. Find the following lines and modify them as follow:
Reboot your device, open the Dropbox app, and log in with your existing account, or create a new account. You will receive an email, and after fulfilling 5 of the 7 tasks, you will receive the 48 GB promotional space. After fulfilling the tasks, you can restore your build.prop to its original state.