Teachers can create a custom canned response for exercises from the "Grade" tab for a given student exercise. At the bottom of the Grade tab is a link to "+ add custom canned response".
Once the custom canned response is created, that response will be available from the Grade tab or Fast Grade for any exercise of that type. For example, if a teacher creates a custom canned response on 1.1.5 Short Stack, it will be available anytime the teacher is grading Short Stack.
Custom canned responses are tied to individual exercises, so that custom canned response the teacher made for Short Stack won't be available for Breakout, etc.
In Fast Grade custom canned responses can be found on the left-hand side under Previous Grades and Canned Responses. Simply click on the drop down arrow and select the response you'd like to add, it will then appear in the message box. You can also use the keyboard shortcut "shift + 1" to speed up the process.