I had the same problem, solved it by:
openning the terminal:
sudo software-center [deb-file-to-install]
in my case:
sudo software-center nautilus-dropbox_0.6.8_amd64.deb
I had the same problem, solved it by:
openning the terminal:
sudo software-center [deb-file- to-install]
in my case:
sudo software-center nautilus- dropbox_ 0.6.8_amd64. deb