If you are presented with the following page :
Firstly you will need to make sure that all the WordPress folders are writeable by the web server (I recommend using 755 for the folder permissions).
You then need to add the following line of code to your wp-config.php file. I recommend adding it below the Database config settings.
define(‘FS_METHOD’,’direct’);
Once you have uploaded this file you should now be able to add / update plugins without entering FTP details.