Did you register the tablet with Samsung? Once you do (and also confirm via an email sent to your Gmail), you will get the $25 Play Store credit.
The registration proces is completely inobvious, at least it was for me. I don't think there was any papers included with my tablet that told me about this at all (and I'm typically very particular about going over everything in the box). The only way I found out even, was good old XDA. The link is here:
http://forum.xda-developers.com/showthread.php?t=2245250
Not sure if the Dropbox bonus is related to the Samsung registration. But its possible the registration may trigger the extra storage, as well. As the previous post, mine took 2 days before I got the storage increase, and I received an email from Dropbox that I got a bonus 48 GB. Again, not sure if it was registering, or simply signing into Dropbox on the tablet that triggered it.