ODM Discover Accounts task no longer has an option to 'Include statistics on mailboxes' as it was moved to Mailbox tab as a separate option.
The option to Collect Statistics is located in the Mailbox section of ODM and can be run against all or a subset of users that were added via the Discover Accounts option.
The following link to the documentation covers the steps to collect mailbox statistics after running a Discover Accounts task is in the Mailbox tab.
