Android emulator sdcard is not writable Read-only file system

I want to copy a video to the sdcard of an android emulator, so that I can play it in an application. When I try to copy the video (by drag and drop) I get the message:

[2013-05-18 23:49:54 - ddms] transfer error: Read-only file system
[2013-05-18 23:49:54] Failed to push selection: Read-only file system

What can I do to make the sdcard writable?
The real problem is that I can't find the directory sdcard in my file system. I can only see it in Eclipse. If I could find the directory, that I know how to change permission (chmod on Mac OS X).

In a terminal (I work on Mac OS X) I wnt to sdk/platform-tools directory (to get access to the adb shell) then I typed the following commands:

./adb shell
su
mount -o rw,remount rootfs /
chmod 777 /mnt/sdcard

and then typed "exit" to exit the adb shell.

I hope it works for others as well, because I cannot find the link to the actual post