First off, the character for escape sequences in Java Strings is the backslash, not the forward slash. But the newline sequence will only be interpreted by text components that inherently support multiline text, and that doesn't include JButton.
Try using HTML with a <BR> tag.
edit Yeah, 5 seconds too late!
edit2: But luckily not another 29 seconds later!
There are no new questions, but there may be new answers.