The autopilot in FSX isn't the best, but it works. I'd start by being 20 miles away from the end of the runway at about 5500 feet. Make sure autopilot is turned on and ILS frequency is dialed in. On HSI, click ILS option. The ILS option should have a box around it, the ILS needles should show up on the HUD, and the ILS identifier and distance should show up on the HUD. Next click the couple button on the up front control panel (a colon should appear before the couple option). Your F-18 should start to maneuver to capture localiser and glide slope. I'd also recommend doing this below 200 kts. Now you can lower flaps and landing gear, if you haven't already. The next step is to modulate the throttle to stay "on speed" (i.e. keep the amber light lit). This procedure works for me. You can try it at different altitudes, approach speeds, and distance from the end of the runway, but being too fast or too close, the autopilot just can't maneuver the aircraft fast enough to lock on to the ILS before you reach the end of the runway. Good Luck!
