Well, you must click on the GWT compiler tool (looks like a red box). After that you must go to Run ->Run as->Web application.
After that it will start doing several things and a URL will be shown. Copy the URL on your favorite browser and delete the part of it that is after the .html
And with that you will be able to check for your proyect.
There are several tips I can give to you:
a) If you're using Eclipse, don't use Google App Engine. It has several problems with Eclipse.
b) Be sure that the GWT developer plugin is installed on your browser.
c) You will have to change the port for your application after the first time you run it. It can be done on Run->Run configurations->Web applications and there you'll find a box that says "Automatically change port number" and that should do it.