After trying everything I know to make internet explorer work with webdriver.io, I've encountered a confusing issue.
To start, download the internet explorer driver from this link: http://www.seleniumhq.org/download/. The file is an .exe named 'IEDriverServer.exe'.
Next, simply save the executable in a convenient folder without running it, and remember where you saved it.
According to the documentation at https://github.com/SeleniumHQ/selenium/wiki/InternetExplorerDriver, the standalone server executable needs to be downloaded from the Downloads page and placed in your PATH.
While there are resources available for setting this up in Java, information on configuring it in webdriver.io seems scarce. Editing the path in environment variables doesn't seem to fix the issue, as one would expect.