I have the Writer Guide for LO 6.0 and have followed the steps to produce a mail merged letter - pages 301 onwards.
I have successfully connected the spreadsheet address file to the document.
However dragging and dropping the field names to the letter simply copies the field names - ie not shaded nor enclosed in < >. If I print the document the system does not prompt me about database fields and the output is a single page with the field names printed.
A possibly related issue is that I do not seem to be able to insert fixed data ie company name and address, which was previously set up using Tools → Options → User Data. These fields do not seem to appear when I use Insert → Field → More Fields