The test project that is part of this widget is build in version 5.19 which is why it tries to open that version. If you want to test it in a higher version you need to create a test project in that version first, then copy the .mpk file to the widgets folder and you should be good to go.
Another option would be to first migrate your test project to a 6.x.x version so that you can migrate that to version 7.x.x
Thanks for the quick info.
I am able to include the widget manually by placing it in my project folder.
But i want to use the feature described in <http://ww2.mendix.com/expert-webinar-developing-widgets.html>
So that i can see the live change in modeler.
Live changes are only possible design time, and in order for this to work you need to use grunt or gulp (or another builder) to automatically build a new widget package after each change. However for this to work with your newer modeller you first need to migrate your test project to 7.x.x via modeller 6.x.x.
So download 6.10.9 then migrate the test project included with your widget then save it and open it with your 7.10.0 modeler.