In the admin cp, left side menu scroll down until you see "Custom BB Codes" click on that and then click on "Add New BB Code". Now you fill in the form like this
Title:
Flash
Tag:
flash
Replacement:
<embed src={param} width=450 height=400 type=application/x-shockwave-flash></embed>
Example:
<embed src={param} width=450 height=400 type=application/x-shockwave-flash></embed>
Description:
Post flash(.swf) movies/games
Use {option}:
No
NOTE: below is optional if you want your members to just click a button instead of typing it out the code then continue on if you dont then leave the field blank
Button Image (Optional):
images/editor/flash.gif
(I created a image for you already for the editor thats if you want the editor to display a button for the user to easily add the code. all you have to do is upload it to "images/editor" folder.)
click save and your done now users can add flash to their posts.