There are many ways a waste dump can be created in Muk3D. You could draw a crest/toe, and then use offset tools to create benches, and then create a surface by triangulating between all the breaklines. Its tedious and time consuming, but you'll still get what you want. But, if you decide to change any aspect of the dump, then you'll need to rebuild it.
The faster option for laying out conceptual designs for waste dumps (or stockpiles, or leach pads) is to use the command found in Dumps/Create/Draw a dump. This command will let you draw the crest of a dump. As you draw, you'll be able to see the dump shell being created, meaning that its incredibly easy to fit a dump to a specific footprint.
The great thing about this command is that once you've finished drawing the dump, a dynamic model is created allowing you to refine the design, either by modifying the crest alignment, or the dump generation parameters.
Once you've managed to fit a dump to sit within your target footprint, find out how to determine the target crest elevation for a given waste storage volume with this tutorial.