Hi Distrum,
Mate have a look at
this video. You are misunderstanding some of the process.
The Select Story or Mechanics Source is just for selecting the Source Topic. Source Topics will be included in all content coming out of the content market. It is intended as a starting / introduction point for all content. This is why you can only select a 'Source' topic in this section.
What you need to do is go into the Partial Export options and create a partial export configuration. You do not select the topics to include in the partial export from this window though. Just set it up with a name and any other details and then click ok to get out of that window and go back to Realm Works.
From there you need to apply a tag to each topic you want to export as part of the partial export. Watch the video for how to do bulk tagging. When you get to the select tag window scroll down to the Export section and you will see a new tag with the same name as the partial export you created above. You need to apply that tag to all topics you want to export.
Once you have applied the export tag you go back to the partial export window and run the export. It will include all topics that have that tag.