I'm using Ubuntu 12.04 with the Gnome desktop from the Software Center. I just installed Ubuntu 1, but it didn't put an icon on my desktop. How do I get one there?
It is my understanding that Gnome desktop doesn't use any icons. If you need to use Ubuntu One you type in the search box "Ubuntu One" and it should be displayed to you.
Here is a nice tutorial on how to enable the desktop icons/right click though. http://joesteiger.com/2011/07/02/enable-desktop-icons-and-right-click-gnome-3-gnome-shell-ubuntu-11-04/
I don't run Gnome personally so sorry I can't be of more help otherwise I would try it out, but maybe this will get you were you need to go.
This tutorial might also prove useful for you http://www.liberiangeek.net/2012/04/add-application-shortcuts-to-your-desktop-in-ubuntu-12-04-precise-pangolin/
Go to your Home Folder then right click on the Ubuntu One folder and in the context menu that pops up you should see an option called "Make Link" if so then click on that option. After clicking that option you should have another folder in your home folder called "Link to Ubuntu One" with a little black arrow in the lower left hand corner of the icon. You can either drag that icon to the desktop or you can right click the icon and go to the "Move to" option on the context menu and select "Desktop". This should accomplish what you asked for, it is a double click not a single click but I don't know how you could change that.