Assuming you have installed PostgreSQL and your WAMP installation is on c:\wamp, you will need to copy
c:\wamp\bin\php\php5.3.9\libpq.dll to c:\wamp\bin\apache\Apache2.2.11\bin
Make sure you also have the following files
C:\wamp\bin\php\php5.3.9\ext\php_pdo_pgsql.dll and
C:\wamp\bin\php\php5.3.9\ext\php_pgsql.dll
Also, make sure you have enabled the above 2 files as extensions, either via the WAMP menu (click on WAMP icon on taskbar, PHP, PHPExtensions, find the above 2 and 'check' them)
Please note that php5.3.9 and Apache2.2.11 refer to my specific PHP and Apache versions.
Adjust those to suit your installation.
That's it!
phpinfo()? Do you get any error messages?