To take a screen shot press fn+F2(possibly F3), I don't remember which). Then click out of minecraft. Click thing that says "go" at the top of the screen. Hold down the option key and a new item appears on the drop down list, library. Keep holding the option key and click on library. Inside you will see a folder called application support. Inside this you will see a folder called minecraft, inside of which you should see a screen shot folder, which contains the screen shot you just took. Hope this helps.

 

that shortcut puts the screenshots in your clipboard, meaning they don't get saved.
You can save the images with Command + Shift + 4 (If I remember correctly, those end up on the desktop.)

 

Looks like you're banned, this is what causes those errors to pop up.

 

We don't lock help support threads, in case someone else is having the same problem.