How do I unlock /var/lib/dpkg/lock?

sudo rm /var/lib/dpkg/lock should do the trick. You can use ps afx|grep dpkg to check if there is still a process runnning at the same time. sudo killall dpkg will stop all running dpkg processes.

If you need to cancel the dropbox installation, try sudo dpkg --force-remove-reinstreq --remove nautilus-dropbox

It means that software center or another terminal is still running a package operation, force removing it is not really recommended as it can cause corruption, but try closing those and if that doesn't do the trick then reboot and just open a terminal window and nothing else to install/do a package operation