1) Musical dynamics such as "mf", or "fff" can easily be entered near any staff line by right-clicking where you want to enter the phrase, then clicking on the "Insert Staff Text" toolbar button:
When the Staff Text dialog appears, simply type the phrase you want:
You may also want to click on the "Text Font..." button to change the font - for dynamics expressions a 14 or 16 point "Times New Roman", bold, italic font looks good.
2) Crescendo / decrescendo can be entered by selecting all the notes to be affected (drag the mouse over the notes while holding the left mouse button down), then click on the "Insert Crescendo" or "Insert Decrescendo" button:
If you want to adjust the size or position of the crescendo / decrescendo after it has been entered, right-click on it once, then drag it to a different location (near where it was initially entered), and/or drag the corners of the dotted selection box to change the size or shape.
3) Symbols such as an accent ( > ) are entered like most symbols in MusEdit: put the caret (the blinking insertion marker) immediately to the left of the note body, then click on the symbols toolbox to enter the symbol, or hit the ">" key on the computer keyboard.
Midi playback of the score is affected by the musical dynamics you insert. Expressions such as "mf" affect the note above (or below) the expression, plus all following notes until a new dynamic is encountered. Cres/Decr. only affect the notes above or below the symbol. Accents only affect the note with the accent.