How to notate low E for guitar

Hello, Brian, assuming the version you are using still works like v. 1.3, you can notate that low f, select it, by simply clicking on it, and use the down arrow key (on your pc keyboard) to move it down to the low e.

You are using notes+tab, and enter notes with the mouse, right? Then the the low f is indeed the lowest, for some strane reason. But if you use the computer keyboard, it works, N, E, Shift+Down, Shift+Down, Esc.

Mouse entry is limited to notes within a certain distance of the staff, otherwise MuseScore thinks you might be trying to enter a note on another staff instead. Mouse entry is never as efficient as keyboard entry to begin with, so consider switching to typing rathert than clicking. Or the Piano Keyboard window, or MIDI input - all of these are more efficient than mouse entry. But if you do use the mouse, just click somewhere that works and use the arrow keys to move the note as necessary.