In the getting started videos there is a video where the speaker is taking a 2D mug that has a top, front, and right-side view drawn up. Each view has a 2D centerline shown. When the speaker is converting the mug from 2D to 3D he drags the outside of the top view upward (switched from drafting view to 3D view) and set his height for the mug. When he did this the centerline he had for the top view (the one he grabbed) was now 3D, having an x, y, and z axis. He then used the centermark for the new top of the mug (just created when dragging up) to center a circle for the inside edge of the mug.
When I did the same thing my centerline did not become 3D, nor did I get a second centermark at the height of what would now be the 3D top of the mug. There's no video or tutorial describing how to make a 3D centerline/centermark, and the way of creating a 2D centerline/centermark doesn't suddenly turn 3D like the speaker's did when he dragged upward.
I think I figured out the issue. You were right about them being drawn in, but I didn't realize that the z axis was drawn in as well. It had been done previously, and when the speaker was in 2D drafting mode it obviously couldn't be seen. When he switched to 3D it was there, but I didn't realize that he had drawn it in. It made it look as if it had just appeared. I also needed 3D object snap turned on, otherwise it tries to snap to the end of the z axis centerline. Thank you for you help.