I'll join the chorus, after a note on this comment, which caught my eye:
"when I try to access it later, it makes me resave it again because it says it's read only."
Are you, perchance, using OS X v10.7.x (Lion)? If so, finding a file locked when you try to access it later could be due to a new feature in Lion, which locks files that have not been used for a while. You can set the length of "a while" in System Preferences.
If that's not the cause, click the link in Peggy's message. It'll take you to Msoft's Mac forums, where you'll find users more familiar with the vagaries of Excel.