There is no need to glue it on.
There are two allen head screws at each side of the spoiler which screw directly into the already threaded pieces at each end of the double spoiler. you will notice two raised sections in the centre of the double spoiler. You screw the screws directly into this section of the spoiler but be careful you dont go too far!! They are self tapping.
If you dont fix it down with these two self tapping screws, when the air temp rises (etc. in the sun) the spoiler will expand and rise, hence jamming against th top of the roof and not being able to open the boot!!
Hope this helps