I want to be able to use SA-learn (Spamassassin learn) to train Spamassassin about mails it has incorrectly identified as spam or non-spam.

1. First, configure MailScanner to store all emails in quarantine as per this FAQ entry

2. You will then be able to perform additional actions on individual
messages or message listings in MailWatch, on any emails that arrive after you've set up storing all emails in quarantine.

To SA Learn a specific email, get the message ID and enter it in the
Jump to Message box in Mail Control, or find the email in the Mail Control listing and click the icon on the left in the "View/Status" column. At the bottom of the page under
Actions you will see several options, including Learn as Ham,
Spam, etc.

You can also use Mail Control to get a listing of messages that match your filter, then click the menu button (top right) and click Bulk Actions. Y ou can then select multiple emails with the check box on the left, and perform the desired action by clicking the appropriate Actions button at the bottom.