The expression field goes with the Type=Expression. This feature allows for direct computation of f(x,y) instead of a fractal function. Note that there is a fractal function available in the expression parser. The available functions are listed in the documentation.

The assumption of the 3D window is that it's viewing a unit cube centered at the origin. If you're using a whole-world top/left/right/bottom of 90/-180/180/-90, then you need to set the Vertical Scale value to something that will get into that range. For a world that's in the 3000 altitude range, try something like 0.01.

One way to get several mounds with the fractal settings on is to draw multiple disjoint selection parts. Draw a selection area. Then hold down the Shift key and draw another. And so on. You can then save that selection as an image file for later use if desired to avoid having to redo your selections.

For the fractal issues, consider increasing the area of the plane evaluated (increasing the Size values on the Height Field Computation dialog). This change will make the size of the fractal elements proportionately smaller and increase the apparent effect on the mound. To get an idea of how it will look as an unmodified noise function, try computing the height field without the mound operation using Filter>>Calculate Height Field. The resulting field will appear as modulation for the mound envelope.

I really should get around to updating things, especially those that predate the great UI reorganization that happened a few years back. You may have noticed that the colors on the height field in the continental shelves tutorial don't match those of Wilbur these days due to a decidedly purple cast. That tutorial is well over 10 years old now, I think. Amusingly, you're the first person to mention that the functions mentioned don't match those in the software.