How to add tags to mail messages in Maverick?
You can do a Spotlight Finder search, and specify that you want to look for mail messages, and then include words from the subject of your email to locate the message you're looking for (e.g. "kind:mail news"). Spotlight will find the underlying file in which the message is stored, and then you can tag it - for example with the word "stuff". Although this might be a bit clunky, you will then be able to use "tag:stuff" in the search box within the Mail app itself, and the Mail will find it.

Even better is if you use Alfred (the app launcher). Someone recently created a workflow for it which enables you to tag messages from WITHIN Mail (i.e. bypassing the Spotlight step) using a hotkey: http://www.alfredforum.com/topic/3383-mavericks-tags-file-and-email-tagging-with-alfred-2-updated-to-v14/

Hope that helps,
Geoff