Activate plugin = Fatal error: Allowed memory size ******

I’ve got WP up and running, installed themes & plugins without any trouble, but when I go to activate a plugin (in this case the wp e-commerce plugin) I receive the following:

Fatal error: Allowed memory size of 33554432 bytes exhausted etc etc.

I’ve done a fair bit of reading about this error message, and have tried adding define(‘WP_MEMORY_LIMIT’, ’64M’ ); to wp-config.php – but the problem persists.