You do need the "#2" file - they split the compressed file into two separate files for those not using torrents. When you pass the name of the first one to 7zip, it will automatically grab the second one in the same directory. Once you've extracted it into the vmdk and vmx files, you can delete the two files you downloaded.

There is no "root" account in the ubuntu environment, you use sudo (password: maemo) if you need to install software there. If you want to install software inside of scratchbox (the device emulator), you use "fakeroot" instead of "sudo" (so, fakeroot apt-get install xournal or whatever)