shortcut #

The # shortcut doesn't have anything to do with accidentals. Shift+3, by default, is the shortcut to change the note duration to a sixteenth note, when entering music in a tab staff. If you have changed the shortcut, obviously I don't know what you've changed it to.

The default shortcuts for accidentals are _ (underscore) for flat, = (equals) for natural, and + (plus) for sharp.

In a score try typing the following:

A - gives you a note in the A spot nearest the previous note. Which A will be set by the key signature.
+, A - explicitly gives you an A#, no matter what the key signature says
=, A - explicitly gives you an A natural, no matter what the key signature says
_, A - explicitly gives you an Ab, no matter what the key signature says

The plus or minus sign is probably an entirely different matter. When a plus or minus sign appears above the end of the measure, that tells you that the measure has been set longer (+) or shorter (-) than the time signature would indicate. This is used most commonly for anacrusis: pick-up measures. If you're seeing this, you may have inadvertently set your note input mode to Insert (icon of a quarter note with a plus sign) rather than Input by note name (icon of a pencil).