As I've said, a user must allow other users to challenge them in order for the option to appear.
If you'd rather have this as an opt-out option instead of opt-in, run these 2 queries:
Query to update current users to;
Allow Challenges,
Use Private Messages and
Someone has challenged you:
Code:
UPDATE user SET arcadeoptions = '41'
Query to auto-enable the same options for future registrations:
Code:
ALTER TABLE user CHANGE arcadeoptions arcadeoptions int(10) UNSIGNED NOT NULL DEFAULT '41'