Why am I getting a white screen when I activate Formidable?

Most likely, you have installed enough plugins that your memory limit has been exceeded. You will need to increase the memory_limit in your php.ini to 64M or 96M. If you're not sure where to find this file or how to change it, please contact your webhost for help.
It might also work to add this to your wp-config.php:

define('WP_MEMORY_LIMIT', '96M');