The sugar demo has sevral files:

28e2a494571154964d05b230595c184a  EfikaMXsugarboot.tar.xz (boot archive)
64663ae12663a1b39e2cfdb618cfc887  EfikaMXsugarroot.tar.xz (root archive)
5986c6b1e475b86aa36efe453e580b25  EfikaSugarDemo.img.xz  (full SD card image)

In order to install just find the name of your SD card device (on linux its likely /dev/mmcblk0) and do xzcat EfikaMXSugarDemo.img.xz | dd bs=1M of=/dev/yoursdcarddevicenamehere 

then just put the sd card in the Efika MX and power on and you should be looking at a sugar screen asking you the color of your XO icon. (you can also hit back to change the name from olpc to something else)

alternatively you can partition a blank sd card into 3 partitions
1 small partition of about 40 to 100MB formatted as ext2 (I think fat32 also 
works but I havent tried it) to hold the contents of the Efikasugarboot.tar.xz 
file, another larger ext4 partition of about 2.5GB (3GB would be better) to contain the EfikaMXsugarroot.tar.xz archive and finally allocate the rest of the space to swap for partition 3.

the process for creating the SD card from the tar archives looks like this 
(replace the mmcblk0 with whatever your SD card device name is)

Step 1: partition the sd card.
fdisk /dev/mmcblk0 (interactively modify the pattitions and hit w when finished)

Step 2: create the filesystems.
mkfs.ext2 /dev/mmcblk0p1
mkfs.ext4 /dev/mmcblk0p2
mkswap /dev/mmcblk0p3

Step 3: Mount the root filesystem and extract the archive to it.
mount /dev/mmcblk0p2 /mnt
xzcat EfikaMXsugarroot.tar.xz | (cd /mnt; tar xvpf -)

Step 4: Mount the boot filesystem onto /mnt/boot and extract the boot archive.
mount /dev/mmcblk0p1 /mnt/boot
xzcat EfikaMXsugarboot.tar.xz | (cd /mnt; tar xvpf -)

Step 5: unmount the volumes.
umount /mnt/boot
umount /mnt

Step 6: Finished!
Just take out the SD card and put it in your Efika and power on. 

or if you did these steps on your Efika MX like me (because my desktop does not have an SDHC reader) just reboot instead. 

Additional notes:
the file boot.scr on partition 1 is used by the uboot boot loader to figure out
that it should do something with that drive and what operations to perform. it is not a plain text file but a binary file made by the mkimage utility. the file boot.script in the boot partition is the plain text boot script. just cd to the boot directory and run ./script-prep boot.script boot.scr to update boot.scr if you find a need to edit boot.script. only do this on the SD card's boot partition unless you know what you are doing or you could make your Efika unbootable (Genesi has an SD card you can use to reinstall your Efika or get a root shell (by answering no when asked to do an install or not if you do screw up somehow)