where do I connect the earth in my pre to?

In some countries the term "earth" is used to mean the same thing that the term "ground" refers to in the USA, which in this case would mean chassis ground/circuit ground/ac safety ground (all of which are usually connected together in audio components that have three-prong power plugs).

So what is labelled "earth" on your preamp is just the ground thumbscrew the others have referred to, that is usually provided on preamps. It should not be connected to the physical Earth.

Some countries, including among others Russia and some parts of Argentina, according to this Worldwide Voltage Guide, appear to have only two-prong ac outlets. That would reduce or perhaps eliminate the potential for ground-loop problems, which is probably what was behind the comment that "the earth connection is for US electrical systems only."

Regards,
-- Al

It's not me using the term "earth"; it is printed on the back of the Lamm, which is a unit initially meant to be sold in the US.

Yes, I understand. Note that I referred to "what is labelled 'earth' on your preamp." I had looked at a photo of the rear panel of the LL2 prior to submitting my post.

What I was implying is that since the term "earth" is used in some non-USA countries to mean the same thing that the term "ground" is used to mean in the USA, and since the preamp was designed by someone whose native country was not the USA, the "earth" designation was most likely intended to mean "ground," not the physical "Earth."

Regards,
-- Al