Can't register a spreadsheet as a data source

I’m trying to set up a mailmerge in LibreOffice Writer, I’ve never done this before so I’ve found a useful tutorial here: https://things.maths.cam.ac.uk/computing/email/LibreOffice5mailmerge.pdf
I can’t seem to register a data source though. Here are the steps I’m following:

Edit → Exchange Database

Browse available databases

Select a spreadsheet (somethinig.ods)
Nothing appears in the available databases section of the dialog box. It is blank except for “Bibliography”.
I’ve tried several spreadsheets.

Ok, that was the wrong place to register a database. Instructions to register a database are here:
https://help.libreoffice.org/Common/Registering_and_Deleting_a_Database