Greetings, all! I know this thread is a little old but wanted to reply anyway. The thing about the "+" icon is bogus. There is no plus icon. On the 'Manage Your Devices' page, you'll see the 'Special Offers' heading as the third one over. First will be your Kindle name, then the Kindle email address, then the Special Offers. Clicking on the 'Special Offers' link will bring up a floating menu box asking what you'd like to do. And that's it.
I'm in the US, but my cousin in Germany was seeing exactly the same page I was when she was trying to unsubscribe, so it's not just about US vs. non-US devices (as far as I'm aware - My billing is US based, while hers is German). More like Amazon simply not being clear in their directions. I was very tempted to call Amazon CS but the invariable wait times kept me from it. Instead I drank a couple beers and everything became clear.
Anyway I hope this helps if you haven't already figured it out.
As to the screensaver, it now shows up as geometric designs, stills and so forth. I'm told one can edit that also, but I haven't tried that yet. Be well.