Clone project into ~/Library/Application Support/Coda/Plug-ins/ (Coda 1.x) or ~/Library/Application Support/Coda 2/Plug-ins/ (Coda 2.x) and restart Coda.

or

1. Download plugin.
2. Unpack it
3. Rename extracted folder to Emmet.codaplugin.
4. Double-click on Emmet.codaplugin to install.

When installed, you will see Plug-ins > Emmet menu in Coda.

1. Go to OSX’ System Preferences > Keyboard > Keyboard Shortcuts > Application Shortcuts.
2. Click on “plus” sign to add a new application shortcut.
3. Pick “Coda” app, enter menu item name with the action you want to change and type a new keyboard shortcut.

You can pick extensions folder in Plug-ins > Emmet > Preferences dialog.

For more info, read http://docs.emmet.io/