I can not access phpmyadmin after installing Lampp(XAMPP for Linux 1.8.1) on Ubuntu 12.04

You need to add this line("Require all granted") to httpd-xampp.conf file at <Directory "/opt/lampp/phpmyadmin"> section.

Here's how the changes will look-

# since XAMPP 1.4.3
<Directory "/opt/lampp/phpmyadmin">
    AllowOverride AuthConfig Limit
    Order allow,deny
    Require all granted
    Allow from all
</Directory>

just type sudo /opt/lampp/lampp security and set password to your phpmyadmin and log into phpmyadmin as username - lampp password - your_given_password

In XAMPP 1.8.1, you have to allow localhost manually by editing httpd-xampp.conf file for screenshots visit this link.