How to enable module when there is a WSOD (blank page)?
Using phpMyAdmin or any other database management software (or via command line mysql if you are comfortable with that), find the name of the module you wish to enable or disable in the system table. Set the status for that module to 0 for disabled or 1 for enabled.

mysql command:

UPDATE  `database_name`.`system` SET  `status` =  '1' WHERE  `system`.`name` =  'module_name';


phpMyAdmin screenshot:



I also recommend installing Drush on your server. This will allow you to execute commands like enabling/disabling modules, clearing caches, etc. from the command line, which is very handy in situations where you cannot access the Drupal admin because of a WSOD.