How do you tell someone on an online dating site that you're not that into them?

A lady waved at me on an online dating site. We got chatting and when she didn't seem to know what Armistice Day was despite coming from Hong Kong, I got slightly suspicious... but still.

The main problem is that I just don't find her physically attractive and while that's not the sole thing in a relationship, it's kind of an essential part.

How do I break it off without upsetting her?