Some metrics require you to provide further information to display them on the dashboard. For example, disk usage metrics require you to specify a mount point (Linux, Mac, FreeBSD) or drive letter (Windows). Other dynamic metrics are network traffic, i/o stats, CPU stats, RabbitMQ and plugins.
You access this panel by flipping the cards round on the dashboard then clicking the metrics button on the left:
You can find out what to enter by looking at your graphs (click the name of the device on the dashboard box heading).
For example, the mount points on the graph below are /, /dev, /dev/shm, /var/run, /var/lock, /lib/init/rw and /boot which is what you would enter into the box on the dashboard.
On Windows, you would enter the drive letter capitalised including the colon e.g. C: or D: