a3vtl.img is a live-system to evaluate the Ardour3 video-timeline. Live-system - you run it from a USB flash drive. It won't touch your hard-disk and does not require installation. It comes with a whole bunch of audio-software and can be used as studio to go, but you're better off to rely on a dedicated distribution for the latter (e.g. AVLinux, kxStudio, Ubuntu-Studio,…).
You need to write the disk-image to a raw-device (USB-stick, CF-CARD,..) that your computer can boot from.
Doing that destroys all data that is on the USB-drive!
Regarding (2.):
on GNU/Linux this can be accomplished by various ways. The easiest is probably to use usb-creator (usb-creator-gtk on Ubuntu or Arch-Linux).
The canonical way is to use dd on the commandline e.g. if your USB-device is /dev/sdb
dd if=/path/to/downloads/a3vtl_VERSION.img of=/dev/sdb
This also works on OSX, disk-devices are called /dev/disk1, /dev/disk2, etc instead of /dev/sdb, /dev/sdc,.. although you may prefer to use the Disk Utility GUI.
On windows: http://www.pendrivelinux.com/ is the probably the easiest way to write a disk-image onto a USB-key.
The disk-image is a read-only live-system. Every time you boot you will have a pristine system. None of your modifications are saved.
In order to retain customizations, you can create a partition with the label 'live-rw' which will automatically be overlaid. The 'live-rw' partition can be on any medium, although it is convenient to keep it on the same disk as the OS. Say, if your USB-drive is 8 GB, you can use the remaining space of 6.5 GB (the live-system is ~1.5 GB) for storage and customizations.
Note: the live-rw partition only saves modifications done to the pristine system. It uses a rather smart technique called AUFS to do so. It is not a data-partition per se and can only be used with a live-system of the same version. If you plan to do serious A/V work, store it on an external hard-disk.
Ardour3 is not [yet] documented. There are bits and pieces of floating around. If in doubt, ask for help via IRC chat.
In the $HOME folder, resides an example video for testing <tt>timecode25-test.mov</tt>, that can be loaded as video into Ardour3:
Launch Ardour3, create a new session and choose <em>File → Open Video</em> from Ardour's menu…
The video-timeline should be pretty much self-explanatory:
#!/bin/bash
# the disk-device to use:
DEVICE=/dev/sdb # or /dev/mmcblk0 or /dev/hdb ,...
URL=http://robin.linuxaudio.org/a3vtl_latest.img
URL=http://rg42.org/d/a3vtl_latest.img
# Download the image and write it to the disk:
curl http://rg42.org/d/a3vtl_latest.img | dd of=$DEVICE bs=64k
### OPTIONAL: ###
# re-read the partition-table after the image has been written
partprobe $DEVICE
# create a 2nd partition using the rest of the space on the device..
echo -e "n\np\n2\n\n\np\nw\n" | fdisk $DEVICE
# ..and create an ext4 filesystem on it, labeled live-rw'
mkfs.ext4 -L live-rw ${DEVICE}2
Now boot from the prepared device..