Most likely cause is that WAMP is binding to an IPv6 address on your version of Windows (ie Vista or Windows 7). To fix this problem simply add "::1" to the "Allow from" in the following file: c:\wamp\alias\phpmyadmin.conf as per the following example:
<Directory "c:/wamp/apps/phpmyadmin3.5.1/"> Options Indexes FollowSymLinks MultiViews AllowOverride all Order Deny,Allow Deny from all Allow from 127.0.0.1 ::1 </Directory>
Be careful with changing that line to Allow from all, which is a common solution found on the internet. This may expose you to a potential risk as the default phpmyadmin configuration allows root access with no password.
Restart the Apache server after making those changes