You interact with a running Virtual Machine via its console which is opened when you start the Virtual Machine. If you have guest operating system installed it will be displayed on the console exactly like the primary OS is displayed on a real monitor.
During guest OS execution all toolbar buttons and many of the menu commands become active. The Configuration Editor is inaccessible.
To switch to the property page click VM Property Page View button
on the toolbar or select Console View in the View menu. To return to console view click Console View button
or select Console View in the View menu.

Guest operating systems can be executed in fullscreen mode when Parallels Workstation menu, toolbar, and status bar are hidden. For detailed information see Switching Virtual Machine to Fullscreen Mode.