I have figured out the answers but am posting this for others, and possibly for myself since I'll probably forget this the next time I go to use SigMaker

In the "Position Image" step, the "Preview on Left" and "Preview on Right" defaults to "E" and "F", respectively. However, if the font you have chosen does not provide "E" or "F", these previews will be empty. Solution: Go back to the "Select Font" step and find some glyphs that exist (are not gray) in the table.
Also, sometimes the "Set Symbol Width" slider "just doesn't work". This may only happen if the Left and Right Previews are empty as described above; I am not sure. Anyhow, the workaround is to switch to another application and back.