First, you'll need to configure TikiWiki to allow smileys. Do this by gong to Admin Home > Text area > General Settings and checking the checkbox for Smileys. (Or use the path your root/tiki-admin.php?page=textarea).
Once you've checked the box, click on Change Preferences at the top or the bottom of the page.
Here's a screenshot:
Text Area Admin Page
Once enabled, you will see an option to add smileys whenever you are in edit mode for a wiki page, article, forum post, blog post, etc. Place you cursor where you want the smiley and click the smiley icon you want to insert. Here's what the Smiley edit option looks like:
Smiley Edit Option
Smileys can also be added by typing a (:smileytag:) wherever you want the smiley to appear. Below are the tags for each smiley: