This is using 8.1 Update 2. Going by this Sitecore doc - https://doc.sitecore.net/sitecore_experience_platform/content_authoring/searching/search_for_an_item - I'm looking at the last section, Add a criterion to a search from the content tree in the Content Editor. I've clicked on the type of item I want, and I click the default Name field to see the dropdown list of the fields in my selected type. I'm trying to click on one of these fields so I can search on it instead of Name, but it won't let me select any custom criteria.
I'm also trying to use the main body search in Content Editor to do the same thing. If I click on the top-level folder and click the Search icon in the main body, I get the Search tab. I click the dropdown arrow and choose Search Filter, the Search by a Field Value. It adds a "custom" tag with a textbox to type, but whatever combo of "field name = field value" I try, it says it's invalid.