I have a 18 horse troybuilt riding mower and cannot find the oil drain plug.

Verticle shaft, look for a small square plug (about 3/8 " on a side) on the underside.

Horizontal shaft: Usually two square plugs, one on two sides above the mounting plate. You only need to use the most accessible one. Sometimes there are two fill plugs as well, you only need to use the most accessible one.