Well, if you do it on the device (that is running harmattan) give the following commands.

Prerequisite; copy your NEMO tar archive to /home/user/MyDocs for example.

## go to root
devel-su

## mount your Nemo partition somewhere to access it
mount /dev/mmcblk0p4 -t ext4 /mnt

## now confirm that you REALLY have the correct partition there, this should give you some info about your Nemo release:
cat /mnt/etc/meego-release

## now, wipe the stuff you have got there:
rm -rf /mnt
sync

## now, extract the archive to partition:
tar --numeric-owner -xf filename+path-of-your-tar-archive -C /mnt/
sync

## now, detach the mount
umount /mnt

That's how to do it. Now if everything went dandies, just boot your device and retry booting to Nemo.