What email service are you using? Is it POP or IMAP?
Different email services will determine a lot on how your messages appear in K-9 Mail. K-9 doesn't have a really extensive Spam filter built into it, like most email apps it's more dependent on the email service that 'feeds' it. Some email services allow you to disable/enable Spam filtering so all messages are put into the general inbox, some like Yahoo Mail allow you to have Spam filtering enabled but the Spam messages are tagged as 'Spam' in the title to differentiate them in the inbox from regular email.
If you're using Gmail, I don't think there is a way to have Spam messages just mixed in the Unified Inbox. Your Spam folder should however always be viewable if you just access Settings >> Folders. If it isn't showing there in the listing, on a computer access your Gmail account in a browser. Click on the 'gear' icon in the upper right and select 'Settings'. The window will fill in with your Settings options, click on the 'Labels' tab, and in the 'Spam' option make sure the 'Show in IMAP' check box is selected.