Hey, sorry that I missed this for one day. As for your questions:
First, there's a box you need to check first before the tool lets you click on the button to load a custom slope shape. There should be an example included in the download but basically, black is the ground/slope, white is air above.
Second, if I understand what you want correctly, the tool cannot do that. The custom tile can only do "top down"-style slopes, because calculating the angle of a non-standard slope is not trivial.
However, the tool is able to do wrap-around circle slopes, they're the right-most slopes. If you want an upside down version of it, just rotate the slope by 180°. If the ground doesn't let you do that, then rotate the input by 180° and make another tile set with the tool, that should give you the upside down version of the inner half circle slope. If you don't quite get what I mean, I could run it for you if you give me your inputs.
So, it's been a while, I'm sorry and I think I got a solution for your problem.
I've been working on another new tool that can make any kind of circle or ellipse shape, since the shape you send me above wasn't actually a half-circle (and I wasn't sure if that was intended or not).
You can find Circle Shape Maker here, for now it's restricted because I'm not done with the page yet, but the tool should work fine. The password is "circle".
Happy using, hope this is what you're after!