Advanced Profile Kit comes preset with the "status wall" content type provided by Facebook Style Statuses added to the variant. If you use FBSS, you don't need to do anything; it will be there automatically. If you don't use FBSS, there are a couple other options for adding a "wall" to your profiles.

Comments on the profile node

If you are using nodes for your profiles, you can make use of the comments on that to make a wall. To do this, go into the "add content" section of the variant, remove the FBSS content type, and add in the comment form & comments.

Guestbook module

The Guestbook module comes with a CTools content type that you can add.

[This needs more detailed explanations as well as screenshots]