You aren't going to have much luck with that widget package at its current early stage of development.
The jar does not include a property JavaBean manifest (the included manifest.mf does not list any of the components), so the palette manager can't find any of the included widgets.
Taking a look at most of the widgets in the package, I see that most of them violate the basic JavaBean spec in any case, so they would not be usable in Designer (most of them are missing the required default constructor). A couple of the widgets (like SimpleCalcPanel, OptionList, etc.) can be used because they supply the required default constructor.
The basic problem with this collection is that it is still an early beta (v0.1.3) where they have focused on widget functionality and not on the details required to make it GUI builder friendly. The package has not been updated since early November, so I'm not sure what the future holds for that collection.
Since the source is provided, you can modify those widgets so that they play well with GWT Designer (and any future bean compliant GUI builder). Here's what needs to be added:
1) Default constructors for each widget
2) Getter/setter pairs for each property that should be set by the GUI builder
3) A manifest.mf file that lists each individual widget (a good example can be found with the Swing MIG Calendar package)