The meatball is almost always set to a 3 degree glideslope, but can be adjusted down to a little below 2.5 degrees.  Never heard that lower angle used operationally.

The meatball tells you where you are vertically on the glideslope so you know immediately if you are high or low on the glideslope.  Too low on the glideslope and you get a ramp strike, usually not survivable.  (The ramp is the rounddown at the aft end of the carrier flight deck)  Too high and you overshoot the landing area. Survivable, but it makes you look bad.  Navy pilots HATE to "look bad around the boat".

The glidslope is set up to bring the aircraft hook to strike the deck just before the 3 wire, causing the hook to engage the 3 wire, an ideal landing.  EVERY carrier landing is graded, and a "perfect" landing score is "OK 3-wire".  Sometimes the hook will hit before the 3-wire but then bounce over the 3 wire and not engage it.  If that happens, it's called a "bolter".  The pilot applies power, climbs, and re-enters the landing pattern (a "go around").   In a bolter, it's unlikely the hook will engage the 4 wire because the aircraft landing gear will rebound before then and the hook should miss it entirely.  Under rare circumstances a 3-wire bolter can result in a 4-wire engagement. This is called an inflight engagement and is BAD because it can result in the aircraft being pulled down to the deck, rather than it flying down to the deck and then engaging a wire and being pulled to a stop.  The hook is designed to sheer off before that happens, but you can never be certain inflight engagments are rare, but they do happen every so often.  But even if the hook does sheer off and you're in blue water ops (far from land with no "bingo" airfield), you're still seriously screwed with no way to get on deck and no place to land.

Centerline alignment is also done visually, but you do that with the centerline stripe painted on the deck.  Since the landing area is angled to the left, and the carrier is moving quickly through the water, from the pilot's perspective, the centerline appears to be constantly crabbing to the right. 

The pilot's scan pattern "in close" is usually:  meatball, centerline, meatball, centerline, meatball, indexer, meatball, centerline, meatball, meatball, meatball.  The indexer is unique to Navy aircraft.  It is a little colored light system inside the cockpit that tells the pilot when he is at optimum angle of attack (AOA) for the approach.  Because the approach is flown on the "backside" of the power curve, AOA determines airspeed, and throttle determines sink rate.  This is opposite to normal flight.  In normal flight the stick controls up and down and the throttle controls airspeed.  To go "up" you pull on the stick and point the nose up.  To go faster, you open up the trottle. 

Flying backside, the stick and throttle functions are interchanged.  The stick instead of controlling up and down now controls airspeed (pull the nose up and you don't go up, you slow down) and the throttle controls sink rate (increase throttle and you don't go faster, you go up.)

Incidentally, there are lights on the nose gear the same color as the indexer lights inside the cockpit.  The landing signal officer (LSO) talks the pilot all the way to the deck and he can see those lights on the nose landing gear.  That way he knows if you are on, under, or over airspeed, very important for a safe carrier landing.

Trivia:  Navy aircraft have a slightly higher failure rate of the right main landing gear relative to the left because the pilot is constanty correcting to the right and so the aircraft tend to hit right wing low.

Hope this answered your questions.  I'll be happy to answer any more if you got 'em.  I'll also gladly answer e-mails, so feel free to e-mail me.