Open your Administration Control Panel.
Click on the Posting tab at the top.
Click on Smilies.
Click on Add Multiple Smilies or Edit Smilies.
Close the ACP menu by clicking on the arrow.
Add/Edit smilies until your heart is content.
Once they are added, go to the General tab and purge the cache.
Then go into Styles tab and refresh the template that you are using.
The new smilies will now be added to your board.
Note: By default the add box is left blank so you will need to make sure that it is selected/ticked.
Tip: By default, the "Display on posting page" is selected/ticked meaning that all the smilies you add will be shown on the posting page and quick reply if you use it. So you will need to unselect/untick this option on all of the smilies that you do not want to appear on the posting page.