How to find which Treemap box is selected.

Not a straightforward thing to but it is possible to get enough information to deduce what the area is.
Unlike the standard charts you can not access the values or xvalues of the series so you have to create code to relate the reported information to your data source.

First as your chart is on a worksheet you will need to create a class object to capture chart events.

Using the events you can capture an index value that relates to the source data.

On the worksheet I have added the arg2 values, G4:H19, next to the source source data, C4:E19. This was done manually but you should be able to create some code that works out the values from the label range.

Use the button, Enable Chartobject Events, to create an object that will capture chart events and report the selected index to a cell, K2.
Click the chart and select a section or subsection of the treemap to see contents of K2 change.