Actually, the dl() function doesn't work on multi-threaded webservers (most Win32 servers) and will just cuase an error. What you'll need to do is set extension_dir in php.ini and then uncomment (remove the semi-colon) from the following line, also in php.ini.
;extension=php_gd.dll
Now restart the webserver and GD should be enabled. Call phpinfo() to verify this.
geoff