There are still a few odd things that are still a bit strange. Dropbox is one of them. Sure, it can be installed form the Software Centre, but often that doesn't add the dropbox icon to the Wingpanel, which means you can't right click on the icon to check status and perform other actions. If does however incorporate Dropbox into the default file manager, where right click actions are available.
Then you get people like me who have their Elementary OS Freya modified. I don;t use the default file manager - I use a personally tweaked Dolphin. And while my Dropbox folder shows up in Dolphin, there are no right click actions for Dropbox available.
So back to the Wingpanel icon. Adding that gave me one click availability for Dropbox.
First, open Software Centre or Synapric (whichever you use) and REMOVE Dropbox.
Then close Software Centre or Synaptic.
Open a terminal and if you don;t already have it - install 'git'
sudo apt-get install git
Then 'git' the installer for Dropbox
git clone https://github.com/zant95/elementary-dropbox /tmp/elementary-dropbox
Then install Dropbox.
I think from memory I got a message asking if I wanted to run Dropbox. Accept, and it will ask for your login details. Once that is done and you go through the initial guide stuff you should have the DropBox icon in Wingpanel. If it is not there, try logging out and in again.
If all went well it will be there. In the screenshop Dropbox is the white icon on the left.