Try to run
apt-get update before you install the openmpi package.
Also you can use the following command to search all openmpi related package, for example:
apt-cache search openmpi
If it still doesn't work, try to use a different mirror, below is GUI and easier:
The package is definitely in the official repository (12.04): http://packages.ubuntu.com/precise/libopenmpi-dev
As mentioned earlier, please run
apt-get update and then
apt-cache search for the libopenmpi packages.
If it still doesn't help. Try to change your mirror. Or paste the content of your