The version of GWT Designer included in the GPE is a smaller version that does not include such things as creating user interface classes from the Eclipse menu.
If you want this functionality, you need to install the full GWT constructor from here:
http://dl.google.com/eclipse/inst/d2gwt/latest/4.3
Please note that I just confirmed that it still allows you to open the wizards you want to create the user interface class, but I have not verified that it works with GWT 2.6. I think that it should work in GWT 2.6.1, but cannot guarantee this. I have successfully used it with GWT 2.5.1.