I have just bought this lens for my Canon EOS 700D camera. How do I activate the macro facility on this lens?
With a tested Minimum Focus Distance (MFD) of 16.81 inches at 270mm and a Maximum Magnification (MM) of 0.26x, or approximately 1:4, you can't really do Macro photography with that lens. And since it is already slow at f/6.3 at 270mm, the minimal gain you would get in terms of MM by adding extension tubes would make the lens too dark to be very useful.
There probably won't be a macro facility to 'activate' - the 'Macro' designation just means the lens can focus very close to your subject. If you are using the lens at the closest range available on the focusing scale (probably using the longer end of the zoom range at the same time) then you can safely claim to be practising macro photography.
There is no official definition of macro photography. For some people, to be a 'true' macro lens you need to be able to obtain 1:1 magnification, that is to say, the image on the sensor is exactly the same size as the object you are photographing, but many lenses are called macro even though they only allow, say, 1:2 or 1:3 magnification.