Shows "unable to locate package unzip" error
Type sudo apt-get update to get the lists updated.
If its the first time you are running this command, it would download something like 20 MB of data.
Then type sudo apt-get upgrade to install the updates, if you want.
You can avoid this step, if you want to install updates later.
After first step finishes successfully, you should be able to install the package unzip by typing sudo apt-get install unzip.
I just had the same problem on a fresh Rack space install of 12.04. The solution:
sudo apt-get install zip -y
This installed both zip and unzip.