I've tried it, and the UI is really clean! Would love to see more features and some quality of life changes :)
Still I don't understand how I should be able to insert a tile into the right side of the program like in the screenshot ._.
Thanks! Feel free to report suggestions on the GitHub page:
https://github.com/mitchcurtis/slate/issues
With regards to tilesets, have you read this page?
https://github.com/mitchcurtis/slate/blob/master/doc/getting-started-tileset.md
It's WIP but it should get you started.