Hi Marcel Pietsen,
If you want to edit the MouseOver you will have to edit the ButtonChrome inside the menu. We use it to render all the button states within our controls. You can find it in the the RadMenu, RadComboBox, most of the RadButtons etc. The RadMenuItems set the IsHighlighted property of their ButtonChrome so if you edit the proper triggers within the ButtonChrome you will affect the MouseOver.
As for the checkbox I am not sure what exactly you want to achieve. In the ControlTemplate of the RadMenuItem there is a visual element that represents the check mark and its visibility is changed when the IsChecked property changes. Eventualy you can replace the visual with a checkbox and TemplateBind the IsChecked of the CheckBox to the IsChecked of the RadMenu
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