Union between two elements

Hello everybody.

I want to make the union between two elements: one is a mesh model of an aircraft imported in Feko and the other is a monopole created with the line bottom in "Construct". 

The problem is that Feko does not allow me to do that. I attach below a photo to show that the button assigned to make the union is not even selected. If someone can tell me why, it would be great. Thank you.