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The big ground pad must be the ground connection. You don't have to ask Lattice. They forgot to mention it in the documentation, but it is obvious.
This link has some information:
https://blackmesalabs.wordpress.com/2015/07/
"One tricky thing about QFNs ( also known as MLFs ) is the substrate ground slug in the center. For many devices – it is the only ground connection and must be soldered – so I place a 0.125″ hole underneath the slug."