The mouse event arguments gives you the LocationF property. It reflects the mouse position in the (2D) coordinate system of the viewport. Viewports in ILNumerics are established by ILCamera. So we have the mouse position in a coordinate system of the parent camera node.
In order to transform the mouse position into the coord system of the plot cube, the transform matrices from the camera to the plot cube node must get accumulated. The resulting transformation matrix will transform any mouse position into the plot cube coord system.
There is a similar question on SO: How to find the 3D coordinates of a surface from the click location of the mouse on the ILNumerics surface plots? 3d-coordinates-of-a-surface-from-the-click-location-of-the-mouse/17681163#17681163