There's a video tutorial in the game.
Just play with all the buttons till you find the right one.OK; 1st, turn on tooltips in the menu.
2nd, click the button next to the picture of the gun, the tool tips should label it "primary fire mode"
3rd, you get a menu with your attack and reaction fire modes.
4th, click the circle next to an option to select a reaction fire.
5th, click the option itself if you want to attack.
6th, click on the alien you want dead
