In order to view the spam directory, login to webmail (yourdomain.com/webmail/) as the main account username and with the main account password.
Then, select Horde to view your webmail.
Once inside Horde, click on 'mail'.
Then, in the upper right hand corner, you'll see a dropdown box. In there, the spam folder should be present. If it isn't, don't worry. It isn't created until the first piece of spam is routed to that folder, so it may not have shown up yet.