You would receive that error if you were unable to connect to the Juju charm store at http://jujucharms.com/. So, you might be blocked by a firewall that is preventing you from connecting to the charm store and so it is timing out, when it tries to download the mysql charm.
You could check that you can connect to the charm store by trying the command:
It should save an index.html file if you can connect. If it times out you have a problem accessing the charm store and you would have to manually copy the charms locally and then use a command like:
juju deploy --repository /path/to/charm-repo local:mysql
If you want to download a copy of all the charms from the charm store for copying to your system then install the package charm-tools and use the following commands:
mkdir -p charm-repo/precise
charm getall charm-repo/precise
If you have no luck and want to debug juju further use the -v flag for verbose output:
juju -v deploy mysql
Hope that helps.