Thanks for letting me know! It will be fixed in the next update, which will come out really soon.
As a workaround, you should be able to make it visible by resizing your (browser) window. Try keeping it as tall/high as possible, and reduce its width. Then the button should become visible at some point. This is only for the embark screen, after that the game should work with any resolution.
Sorry for the issue, I hope you'll give it another try!