I installed Apache and then PHPMyAdmin on my Ubuntu server, but I have not been able to access PHPMyAdmin at
http://localhost is working however.
You'll need an Ubuntu server with initial setup and a LAMP Server
If you haven't installed the LAMP Server. You can follow the steps on the below link to install it.
Then Install the phpMyAdmin in your server. Follow the steps on the link below.
Did you install MySQL?
Another possibility is to download phpmyadmin and extract is to htdocs/phpmyadmin.