You will need to add the corresponding types to the Preferred Types Editor. For this case that means adding the visualElement class type you are using, as well as the MouseDownEvent class type. Once both are in the Preferred Types Editor, you should be able to see the visualElement type in the Reflected category and use its methods.
You are right. Because of the constrains the RegisterCallback has it is not handled correctly and an error is poped. I will need to correct this. For a quickfix however, you can open up ReflectedMethodNodeWrapper.cs file and comment lines #57 to #61 in the SetMethod method. Doing this will allow you to add the RegisterCallback method node. Just remember to right click on it once it is added and from the menu select “Change Generic Type” (to MouseDownEvent in this case).