I'm not familiar enough with TeensyDuino to tell you exactly what you need to do. I see in the wiki they say:
The instructions here will help you setup the development environment using Arduino´s IDE version 1.0.6. Currently working in getting it going on Arduino 1.6.5.
This tells me that the boards.txt file is in the old format. The easy way would be to just use the Arduino IDE version the thing was written for but that's not to say it's impossible to get it working in Arduino IDE 1.6.8(actually I recommend 1.6.9 since 1.6.8 had a fairly serious bug). There is a guide on how to convert from 1.0.x format to 1.6.x format here:
The problem is that it looks like teensyduino has a slightly different format. I might be able to infer what changes are necessary if you post the original teensyduino boards.txt file.
boelle:
Board teensy:avr:cdcsav doesn't define a 'build.board' preference. Auto-set to: AVR_CDCSAV
Board teensy:avr:usbtsav doesn't define a 'build.board' preference. Auto-set to: AVR_USBTSAV
Board teensy:avr:avr1091286 doesn't define a 'build.board' preference. Auto-set to: AVR_AVR1091286
Board teensy:avr:usbtiny1286 doesn't define a 'build.board' preference. Auto-set to: AVR_USBTINY1286
Those are nothing to worry about. They are annoying but are only warnings and can easily be fixed.
boelle:
exec: "C:\Users\Bo\Desktop\Arduino IDE\arduino-1.6.8\hardware\teensy/../tools/": file does not exist
That's the real problem and I don't see anything in the modified boards.txt that would cause it. I think reinstalling the Arduino IDE and Teensyduino is the best solution in case . Make sure to install v1.29 that supports Arduino IDE 1.6.8.