When you add anouther volume to the instance (using amazon's web gui "attach volume") it's as if you plugged it into the machine. You should see the new drive using
You'll need a filesystem on it. Lets say the new volume appears as /dev/xvdg, so
mkfs.ext4 /dev/xvdg - (this is assuming you don't want to upt partitions on that volume, that's a whole other topic).
You'll then need to mount it, either once off or long term. To do it long term, make a directory, and put an entry (a new line) in /etc/fstab. It'll look something like:
/dev/xvdg /mnt/point/you/made ext4 defaults 0 0
save the file, then try mounting with
mount /mnt/point/you/made - doing it in this order makes sure the mount command uses the fstab file for its information, which ensures the fstab file is ok.
It goes without saying, don't mess these steps up (like formatting the wrong disk!), or you can lose data. In particular, don't reboot until you've made sure your fstab file is ok, or your machine may not reboot properly. (you can fix it, but it's a real pain, particually in amazon ec2).