@alexsh1 I did add the link in the preference page, but didn't add the board after that. Thanks alexsh1!

Hello,
I'm using a Macbook (2015) running HighSierra (with latest updates). After installing the Arduino IDE (1.8.5) I can communicate with a Sensebender. I don't know how to add the board definitions for it and MySensors GW. On a W7 machine I can locate a boards.txt in the hardware directory, but none of suchs on the Macbook. The Arduino website has loads of guidance for Windows, but extreme little for Mac's. When I download ArduinoBoards-master, Safari unpacks in the Downloads directory... I've tried to make a hardware directory under Arduino, but that gets ignored. What is it that I'm missing here? Any suggestions?

I took a drastic measure, uninstalled the whole lot, deleted all libraries, rebooted the PC and re-installed Arduino IDE, 1.6.12 now, installed MySensor-Master.zip, added the SI7021, ash204 and RunningAverage libs, from https://github.com/mysensors/MySensorsArduinoExamples/tree/master/libraries and guess what, still all the familiair error messages.
When I took all the SI7021, ash204 and Flash items out it compiled nicely...
My next challenge will be to add DS18B20 temp sensors, add the furnace controller logic (which works nicely on an Arduino Uno on the moment) and make it all visable in Domoticz.

I've tried your suggestion, but errors keep comming. There is no keywords.txt there.
Then I did a Google search and located 4 differtent versions, tried them all, but every time at linking time I keep getting errors like I had.

I recently bought 2 Sensebender Micro, they worked straight away with FW version 1.1.
When I download the sourcecode 1.4 and try to compile it, all seems to go OK until the linker is invoked, I get the folowing message:

Arduino: 1.6.11 (Windows 7), Board:"Arduino Duemilanove or Diecimila, ATmega328"

Linking everything together...
"C:\Users\Gregory Weber\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\4.9.2-atmel3.5.3-arduino2/bin/avr-gcc" -Os -flto -fuse-linker-plugin -Wl,--gc-sections -mmcu=atmega328p -o "C:\Users\GREGOR~1\AppData\Local\Temp\buildd219cddef0aa3a62d167b7fb84069ff4.tmp/SensebenderMicro.ino.elf" "C:\Users\GREGOR~1\AppData\Local\Temp\buildd219cddef0aa3a62d167b7fb84069ff4.tmp\sketch\SensebenderMicro.ino.cpp.o" "C:\Users\GREGOR~1\AppData\Local\Temp\buildd219cddef0aa3a62d167b7fb84069ff4.tmp\libraries\SPI\SPI.cpp.o" "C:\Users\GREGOR~1\AppData\Local\Temp\buildd219cddef0aa3a62d167b7fb84069ff4.tmp\libraries\Wire\Wire.cpp.o" "C:\Users\GREGOR~1\AppData\Local\Temp\buildd219cddef0aa3a62d167b7fb84069ff4.tmp\libraries\Wire\utility\twi.c.o" "C:\Users\GREGOR~1\AppData\Local\Temp\buildd219cddef0aa3a62d167b7fb84069ff4.tmp\libraries\RunningAverage\RunningAverage.cpp.o" "C:\Users\GREGOR~1\AppData\Local\Temp\buildd219cddef0aa3a62d167b7fb84069ff4.tmp/core\core.a" "-LC:\Users\GREGOR~1\AppData\Local\Temp\buildd219cddef0aa3a62d167b7fb84069ff4.tmp" -lm
C:\Users\GREGOR~1\AppData\Local\Temp\ccf8ZNat.ltrans2.ltrans.o: In function `sendTempHumidityMeasurements(bool)':

ccf8ZNat.ltrans2.o:(.text+0x1d0): undefined reference to `SI7021::getHumidityAndTemperature()'

C:\Users\GREGOR~1\AppData\Local\Temp\ccf8ZNat.ltrans2.ltrans.o: In function `setup':

ccf8ZNat.ltrans2.o:(.text+0x462): undefined reference to `SI7021::begin()'

ccf8ZNat.ltrans2.o:(.text+0x508): undefined reference to `SI7021::begin()'

C:\Users\GREGOR~1\AppData\Local\Temp\ccf8ZNat.ltrans3.ltrans.o: In function `global constructors keyed to 65535_0_SensebenderMicro.ino.cpp.o.2482':

ccf8ZNat.ltrans3.o:(.text.startup+0xd8): undefined reference to `SI7021::SI7021()'

collect2.exe: error: ld returned 1 exit status

Bibliotheek SPI op versie 1.0 in map: C:\Users\Gregory Weber\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.6.14\libraries\SPI wordt gebruikt
Bibliotheek MySensors op versie 2.0.0 in map: C:\Users\Gregory Weber\Documents\Arduino\libraries\MySensors wordt gebruikt
Bibliotheek Wire op versie 1.0 in map: C:\Users\Gregory Weber\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.6.14\libraries\Wire wordt gebruikt
Bibliotheek EEPROM op versie 2.0 in map: C:\Users\Gregory Weber\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.6.14\libraries\EEPROM wordt gebruikt
Bibliotheek RunningAverage in map: C:\Users\Gregory Weber\Documents\Arduino\libraries\RunningAverage (legacy) wordt gebruikt
exit status 1

So far my Domoticz server is running on a Raspberry Pi 3 and I'm using an Arduino Uno as serial gateway, it didn't run on an Arduino Ethernet with softSPI for some reason.
I have limited programming experience, let alone what goes wrong when libraries start yelling at me.
Can someone guide me how to solve this?