Hi, thanks for your questions. I see three things that will need changing: your leave-stock, cut depth, and min-depth are too large.
A quarter of an inch leave stock, for that particular project size and image means almost all of your project will leave nothing to be cut because the leave-stock effectively expands the project surface in all directions. If you have a perfect hemisphere,that is 0.5" in diameter, for example, and have a leave stock of 0.5" then the actual surface manifold which toolpaths will contour are limited to a 1" diameter hemisphere. The leave-stock imposes a margin in all axes, not just the Z axis. This way any vertical parts of the surface will also push the tool out away to ensure that there's stock left there. Start with a leave stock of zero just to get things working and increment it from there as you see fit.
Secondly, the cut depth is going to result in only two sets of horizontal cuts being generated. The horizontal milling operation generates sets of concentric cuts at 'cut depth' increments up to and including 'max depth'. So right now with 0.25" cut depth and project Z size of 0.5" it will try to generate cuts at the vertical middle of your project and then again at the bottom. The particular image you're using doesn't require much material to be removed as most of the features are all just below the top surface, with just a few pockets that actually go below 0.25", and with the 0.25" leave stock you had they pretty much just get removed entirely as cutting into the pockets would mean that 0.25" of stock isn't left in there between the cut and the surface your image produces.
The minimum depth parameter sets the depth-wise starting point for the cuts, relative to the top surface of your project (the max-depth similarly confines cuts to a maximum depth below the top surface). The minimum depth parameter functions as a cut-off point above which no cuts will be generated (except for engage/entry cuts), and with 0.25" you're preventing any cuts from generating in the top-half of your project. Set it to zero unless you have a reason down the road for cuts to be confined to the deeper parts of a project.
To help you get started I'm going to suggest trying a leave stock of 0.0, a cut depth of 0.1, and a min.depth of 0.0. Also, you might want to use a smaller step size, usually half your cutter diameter is perfect for horizontal operations. Start with that to get something happening and then play with changing different values here and there so you can get a feel for the way things are setup.
Not exactly sure why your simulation isn't showing anything - unless that's a screenshot from when there were no toolpaths generating (none are displaying so I imagine that to be the case).
Let me know how it goes!
Charlie