[Serpentine]

Nah, there's a nice easy way to do it without the plugin - I've never use the plugin.
I was going to do it manually, but using a script is far easier.

Anyway, assuming you have Python installed here's the script:
Right click me -> save as

1. Save that to the folder which contains all your books - Don't worry if your books are in subfolders, it will crawl all the subfolders too.
2. If your python is using the default settings, and you have .py files associated: Just double click it, this should run it, a file called "bookisbnlist.txt" will be created in the folder.
3. Open the text file.
4. Swap to Calibre. Add Books (in the top menu bar) -> Add from ISBN
5. Copy and paste in the content of that text file.
6. Add them.

If you don't have Python, or don't have the files associated - figure it out
The python is written in a rather readable manner - so if you want to try other patterns, you should be able to edit it easily enough.