The thing about the guestbook is, it's not a feature of MSN Spaces, it's a cunning use of the blog and list features.

Basically, do the following:

1. Create a blog entry called: Guestbook
2. In that entry, type a message that invites people to leave you a comment.
3. Publish that blog entry
4. Preview your space
5. Click on 'Permalink' at the bottom of the Guestbook blog entry.
6. Copy the Permalink link for later use.
7. Create a new custom list called: Guestbook
8. Add one entry to the list called: Leave a message (or something similar to that).
9. Set the link for that list entry to be the permalink you copied in stage 5.
10. Customise your space and place the list called Guestbook wherever you want.

To start with your Guestbook blog entry will be on your front page, but as you add blog entries it'll become hidden and all you're left with is a list that links directly to it.

Read Permalink Tutorial for better understanding.

A special thanks to Wesley for this manual.