How do I install it?

[nearestnabors]

I'm looking on Google and in the Readme, and in the help menu, but I can't figure out how to install it :(

[nicolasembleton]

You can just open Zed, go to Extensions, search for "Syntax", and click install.

Edit: I realize that maybe when you asked the package wasn't in the Zed repo and thus invisible in the list of extensions.