Sorry for OT, does this mean our Bluetooth keyboards and mice will stop working soon as well ?
No. What I was referring to is the new-generation low-energy Bluetooth hardware that's just become available to engineers in the past year or 18 months. Apple is leveraging that to provide proximity-sensitive secure automatic connectivity between devices. (That's why things like AirDrop between the iPhone in your hand and the Mac on your desk only work when they're no more than a couple meters apart.)
The new hardware is backwards compatible with older Bluetooth applications. So a new Mac will work with your old keyboard. In that way it's similar to the new Macs' (and iDevices') fancier newfangled WiFi capabilities that will yield greater speeds with latter-day WiFi access points but still connect to older ones. (Although, maybe that's not the greatest analogy, as IIRC Apple recently dropped support for WEP encryption, which while terminally insecure might be all that's supported by really ancient WiFi hardware.)
Anyway, there's no reason to fear your old hardware will stop working.
In addition, the days are mostly gone when peripheral manufacturers fail to support new Apple hardware and operating system revisions. I sorely remember some marketing troll at HP saying people should just buy new printers if they wanted compatibility with OS X Lion. That attitude convinced a lot of people, including me, to avoid HP hardware from that point on. They seem to have come to their senses in that regard, fortunately.
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I've downloaded Dropbox but it's not showing up in finder?
The download results in a disk image (.dmg) which you will need to double-click. It then appears (mounts) like a disk drive on your desktop. Double click the "disk drive" and it will open. Follow the instructions for running the installer it contains.
If you've downloaded the .dmg and can't find it, try using Spotlight to search for ".dmg" Usually downloads end up in your user-account's Downloads folder, but maybe your browser is configured otherwise?
https://www.dropbox.com/install is the link from which you can download the installer .dmg.