State tax filing

The answer to your question is that an item of income like this (honorarium) gets reported on Form 1040 Line 21 as "other" taxable income (even where you didn't receive an official tax form).  It's very much like reporting miscellaneous compensation for Jury Duty.  The facts that you actually performed your services in Mexico, and that a California academic institution was the original source of your $400 payment, actually are not material to the correct answer tax here.

This is because you were not living abroad long enough (one year or more) to qualify for any foreign earned income exclusion; and because your California-source income ($400) is not high enough to meet the income tax filing threshold in California.

In TurboTax, the mechanical instructions for entering an income item on Form 1040, Line 21 are as follows:

To begin, you will want to have your tax return open and locate the Search / Find box on your screen.  Next, type in the exact search string "other reportable income" and then click on the Jump To link that should appear beneath.  This will take you to the appropriate data entry starting place in the program.

You will then need to pass through a few sequential screens asking questions not relevant to you.  But, when you reach a screen titled "Other Taxable Income" (or something similar, depending on your version of TurboTax), you can enter your honorarium item there.  You will then continue on for another couple of screens, and you will eventually exit this portion of the program . . . but with your honorarium income item now entered.

As an example of this, and as a guide to help you navigate, I've created a series of screen-capture images, which you will find directly beneath this text (simply click the images to open).  If you follow them carefully, and make the entries that I did in my test file, you will achieve the same outcome.  An amount of $400 will appear on Form 1040, Line 21, as ordinary taxable income (but not as income subject to additional self-employment taxes).