It looks like you are either not using a Macintosh or you are using a version of macOS older than 10.12 (Sierra). You will need to upgrade to be able to run Swift for Arduino.
Version 2.0 is out! This is a bringing together of many different strands of work from library upgrades to IDE revamps. Looking good!
Version 1.8 is out! Some amazing new features here. Many, many thanks to Mark Swanson for his tireless, creative and professional contributions, the IDE feels like a new product. We make a great team!
Version 1.6.1 is out! More bugfixes to syntax highlighting (beta).
Version 1.6 is out! With support for globals in files other than main.swift and some small bugfixes to syntax highlighting (beta). Enjoy!
Version 1.5 is out! Contains a few bugfixes, some simple indenting and a beta of syntax highlighting. Much thanks again to Paul Shelley for his great work on syntax highlighting, we hope you love it!
Version 1.4 is out! I found that the AVR backend currently has occasional optimiser issues. If your code is behaving unexpectedly with optimisations turned on (which is the default) then try going into preferences and turning it off. This version makes sure that turning optimisations off only affects the last stage of the pipeline (the bit with the issue) so you'll still get much of the performance benefits anyway. Later versions will attempt to track down and fix the optimiser issue.
Version 1.3 is out! Containing bugfixes for the Arduino detector contributed by Paul Shelley, as well as large font mode and new library functions to use the internal arduino temperature sensor and band gap.