Why is the CiviCRM installer unable to connect to the MySQL server?
In my experience if 'localhost' (or your host) was refusing to connect on default port with all credentials/permissions definitely correct, specifying 'localhost:3306' will work (even though you shouldn't have to specify the :3306, and I think it tells you as much).

I will also note that this is something I have only experienced recently, and with this question there are examples on Ubuntu, and in my case MAMP.