I am developing GWT application. In dev mode, I can successfully open the app in FF and IE. However, when I try to open it in chrome, I get the missing plugin window. After
Do you see the GWT toolbox in the omnibox? (address bar)
Have a look here for more troubleshooting tips: http://code.google.com/p/google-web-toolkit/wiki/TroubleshootingOOPHM