This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I think the problem is related to the theme you have inherited. are you using any theme?? ( check your module.gwt.xml)
Also, use firebug to inspect the dialogbox element. The dialog box is right here but the styles are creating issues.