I don't think the forward-slash makes any sense in your Metadata Plugboard expression as it is referring to directory creation and filenames.
What happens if you change the Plugboard expression to
Code:
Dr Who {#dr} - {series}{series_index:0>3s} - {#seriesx}{#seriesx_index} - {title}
For the Send-to-device expression have you double-checked what's actually in the #mainseries custom column, by displaying the column in your main library window?
By the way, I don't have a Nook so please feel free to ignore me