Open a WebOffice 10 R3 project using the WebOffice flex client by clicking Flex viewer in the corresponding line of the Projects tab in WebOffice 10 R3 Administration pages.

WebOffice 10 R3 Administration pages - 'Flex viewer' button
You can run a WebOffice 10 R3 project with the WebOffice flex client using the client=flex parameter in the HTTP query string as well:
http://server:8080/WebOffice/synserver?project=flex_vertical&client=flex
Note: The WebOffice flex also supports the usage of compact caches.