How to run a package with rosrun

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@gcdhoorn: and of course finally to that topic:

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

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

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

I am sorry...