@gcdhoorn: and of course finally to that topic:

I try to: rosrun myahrsdriver myahrsdriver _port:=/dev/ttyACM0 now

but: 1.) where to start that command? "catkin root" or where the myahrsdriver file sits: catkinws/devel/lib/myahrs_driver

2.) no matter where I run "rosrun" it cannot find the package.

I am sorry...