You need to set a ground attack way point on the target, and then click ‘set’ I believe. This should make a bright green crosshair appear which you then click on the target you want, i.e a particular tank or such like. This should then be set and ready to go.
Also i would make sure you have ample height as well so that the Ju88 bomb can function properly ( and to minimise the chances of the attack not working from being too low etc)
Hope this helps 🙂