In Scrapebox - Addons - Show Available addons
Click on the Scrapebox Rapid Indexer line and in the panel below you will see an option to "Click here to download a small list of indexing sources"
Download this list (called sources.txt) to anywhere in your Scrapebox directory / subdirectories.
Then go back to the Addons menu and activate Scrapebox Rapid Indexer.
At the bottom of the screen there is a downward arrow labeled Indexer Services. Click this and another panel opens with an option to "Load"
Use this to load the list you downloaded (called sources.txt)
Now there are two ways to add more of these sites.
1. Use copy/paste to insert more sites in the panel where you loaded the sources.txt file paying attention to the format used by the existing list
2. Or edit the sources.txt file direct and paste to insert more sites file paying attention to the format used by the existing list
You can also replace the existing sources list in the panel by loading the new file, "PR1Up_UseThis.txt", which was kindly provided by the op of this thread.