- First of all, be sure to use the Arduino.cc IDE and not the Arduino.org IDE. At the time of writing you can distinguish them in the version number; you'll need the 1.6.x releases and not the 1.7.x releases. It might seem like you're using an "older" version but that is not the case; they are just different branches of a very similar IDE.
The biggest difference is that the Arduino.org IDE at the moment does not have the simple "Board Manager" user interface to enable other boards than the standard Atmel AVR boards.
- Then, use the "Board Manager" to install the "Arduino SAMD" toolchain. When starting the board manager, I received a "package_index.json file signature verification failed" error message.
Not sure what caused this (remnant of my previous 1.7.x Arduino IDE installation?), but I had a remnant package_index.json.sig file in the Arduino AppData folder (%APPDATA%/Local/Arduino15/). After deleting that file, I was able to search and install in the Board Manager correctly.
After installation of the Arduino SAMD Boards support, you can now select the MKR1000 as one of the boards in the IDE and start fiddling around! Hurray!