SpamAssassins spam box works under IMAP so you need to login to webmail with the specific email account to view spam folder.
Email that are flagged as spam by SpamAssassin is placed into the "spam" folder on a user's IMAP account.
The purpose of "subscribing" in Squirrelmail (note this is not required in Horde) is to allow you to view the contents of that folder. It's a weird Squirrelmail thing, if you use Horde or an IMAP client to download mail you won't need to deal with this oddity.
In Horde (or any other webmail client if you installed any), the spam will simply be in the "spam" folder on each user's account.
