In real world, I can set an OBS on the CDI and the  GNS430 shows the magenta/white line.

1. Set a direct-to, let's say an airport

2. Press OBS-button

3.With the OBS button you can now turn the magenta/white line as needed, for example adjust it to the runway.

Same procedure is used for a holding

But this seems not to work on my Sim.

Is there a setting needed for this ?

 

Hi,

Yes there is a setting for this, documented in the "Settings" chapter of the RXP GNS User's Manual.