Super,
I'll try!!
As you say, you have a series of indicies. These are reference a particular datum. By dividing your total index (sum of all indicies, which will be in the form of KGM, Foot-pounds or whatever system you are using) by your all up weight, you end up with a distance. This is the CG as a distance from the datum. You therefore meed to know where the datum is (it is typically, but not exclusivley, the nose of the aircraft.). This will almost certainly (no, make that CERTAINLY for any conventional aircraft) fall somewhere on the wing.
MAC is simply the avegage chord. For a constant chord wing, it IS the chord. For a constant taper wing, it is simply the average of the root and the tip chord. Sweep back makes it a little challenging, but draw a couple of wings and a graphical solution will become obvious.
If the CG you've just established falls 1/3rd of the way back from the front of this mean chord, your %MAC is 33%. If it's half way, it's 50% and so fourth.
Typical ball-park figures for a jet transport would be 10-40% MAC.
So, the parameters you need are Zero Fuel Weight, Zero Fuel index, index for each loading zone, index for fuel, and position of the leading and trailing edge of the MAC reference the Datum.
Hope that helps!!