As of April 2013 iTunes Connect seems to have stopped accepting Unicode characters directly.
However you can still enter them as HTML character entities (a code recognised by things which render HTML – like browsers and iTunes, and the App Store app on the device).
To find the “HTML Entity” code you ideally need to know the exact name of the character; have a look here for clues: http://en.wikipedia.org/wiki/List_of_Unicode_characters
Then just Google “unicode (name of your character here)” for example: “unicode black star”.
That will find you this page, where you can see the HTML Entity code that you want is: ★ (or ★)
So to get a star (★) you write: ★
And Five stars (★★★★★) is: ★★★★★