But did you check the button config to seek for the right command?
Try this for the f15: start a mission in the air and press 4, make sure you have a missile selected (green box around the name) in the Programmable Armament Control set or PACS (the mfd that shows the payload). Then press RALT+SPACE for a few seconds until the missile fires.
Right ALT not any ALT + ENTER for a few seconds.