In order to define a contextual action for marker nodes, you need to put your Python script in your Python directory under a directory called Contextual_Well_Marker_Node.
Go to Tools->Options->Environment->Python Scripts (or Tools->Options->Python->Scripts in INTViewer 5.2) and click "Refresh from Disk". Your script should be listed automatically.
You will now be able to execute this script from the contextual menu of the marker node:
This script will list all the curves, deviation and log arrays of the data that intersect the marker, and the value or values at that measured depth for each in a message dialog.
This script is attached to the bottom of this article.
The result after the script executes should look something like the following.