You don't enter notes with the Trim tool, you do it in the Piano Roll.
Remember, the Trim tool isn't specifically for adding blank space, it's for adjusting the length of a Region in either direction. So, for example, if you have, say, a Region that you want to Loop with the Loop Pointer and it's not the right length to loop and stay in time, you can adjust its length [shorten or lengthen it to make it an exact number of bars] with the Trim tool.