I was working along just fine in dynamesh and then it started subtracting the elements I was adding. I figure i must have set something along the way, unknowingly. Thanks.
either you have pressed zsub (instead of zadd) or you had zadd on correctly, but you have pressed the alt-key while inserting the mesh. it first looks ok, but when you dynamesh it substracts. you can see which meshes are substracted, by turning polyframe on. the substracted meshes will be white. the added will be any other color.