I was wondering whether a dash should be written in MLA as two keyboard "minus signs" aka hyphens (--) or use the actual special character for a dash, typed on Mac with alt + minus sign/hyphen (–).
A dash should be written as an actual dash, but the usage of two hyphens to approximate it goes back to the typewriter era and is so common that has its own section on Wikipedia's dash page. If your teacher hasn't made a big deal over using the proper formatting, it's probably perfectly acceptable to use two hyphens together. On the other hand, to be safe, you could ask them and, if you already know how to type a dash, there's no reason not to.
Be more careful to distinguish between en and em dashes, however. Your Apple code only gives you en dashes but the punctuation that replaces commas, parentheses, and colons should more properly be an em dash. (The MLA's website doesn't discuss the distinction but does prefer dashes without spaces and uses em dashes itself.) To type it on your Mac, add a
shift key to the code for the en dash. (You can also look through Diserasta's list to make sure you're not misusing it to do a hyphen's work.)
The MLA also uses the rarer 3-em dash for repetitive entries in the Works Cited section. Their website notes that they accept 3 hyphens in submissions, but convert them to the proper character before publication. Their website also goes out of its way to tell you not to second-guess or argue with your teacher:
These comments refer to projects in which the decision... is up to you. In a classroom, the teacher usually has the final say on the formatting of assignments.