TDC=Top Dead Center, or in other words, when the #1 cylinder is at it's topmost position during the compression stroke. You can find the timing marks down on the front pulley and front cover of the engine. What the instructions meant was if you rotate the engine until the line on the front pulley lined up with the TDC mark on the front timing cover, the rotor in the distributor should be pointing at the #1 plug wire/tower of the distributor cap. Mind you, at TDC the rotor can actually point at the #1 or the #6 wires since the distributor rotes once for every two engine rotations. This of course leaves you with a 50/50 chance of getting it right. There is a way to be sure, but it's much quicker and easier to just try the two wire combinations.