in the middle icon of your interface at the bottom of your screen you should see your current chosen weapon and the AP amount it takes to shoot it. If you RIGHT click on it, it will change modes from normal shot, targeted shot (shown by a crosshairs), or reload. Once you change it, then left click, it will perform whatever action you have chosen.
To get the option of picking a body part it has to be showing the little crosshairs icon.
But having your eyes damaged will hurt your accuracy, so try to find a doctor somewhere who can fix that up for ya, or have someone in your party use a doctors bag (assuming they have some ability)