I know I'm supposed to enable text replacement via Edit > Substitutions > Text Replacement but… I cannot find that menu in Chrome at all. Help?
This is currently not possible because Chrome does few things its own way. Here is the bug that tracks this
Still if you dream about adding a comment there forget it, even if Chromium is Open-Source the team decided that they don't need external feedback and disabled the commenting.
There is also a thread on Google Forums about this bug. But again, Google closed the topic in order to avoid other comments.
I guess that your only option would be use Twitter to tween about this issue. Be use that you mention
That feature appears to be specific to Safari on OS X; however, there are plenty of extensions for Chrome that imitate that functionality (https://chrome.google.com/webstore/detail/auto-text-expander-for-go/iibninhmiggehlcdolcilmhacighjamp?hl=en for example). It will not use the built-in OS X dictionary, unfortunately, but can sync dictionaries across Chrome browsers on the same account.