When using the Sublime Text plugin...
Use Control + E as global trigger for expansion even in files such as codekit's new include.kit files
see documentation:
https://github.com/sergeche/emmet-sublime/blob/master/README.md
Tab key handler
Emmet plugin allows you to expand abbreviations with Tab key, just like regular snippets. On the other hand, due to dynamic nature and extensive syntax, sometimes you may get unexpected results. This section describes how Tab handler works and how you can fine-tune it.
By default, Tab handler works in a limited syntax scopes: HTML, XML, HAML, CSS, SASS/SCSS, LESS and strings in programming languages (like JavaScript, Python, Ruby etc.). It means:
You have to switch your document to one of the syntaxes listed above to expand abbreviations by Tab key. With Ctrl-E shortcut, you can expand abbreviations everywhere, its scope is not limited. When you expand abbreviation inside strings of programming languages, the output is generated with special output profile named line that generates output as a single line.
See also the main Emmet documenation: