@Lucio looks like it is installing , but all you needed was just the first "sudo add-apt" to run in terminal, or to add the sources in the list (with gksudo probally better than sudo on opening gedit) so the sources look like this :
deb http://ftp.osuosl.org/pub/mariadb/repo/10.0/ubuntu precise main
deb-src http://ftp.osuosl.org/pub/mariadb/repo/10.0/ubuntu precise main