If you are using ISBoxer you need to tell the software to pass the key. If it is an FTL command just copy one of the DPS keys and change the binding to one of your mouse buttons.

The same can be said for Keyclone if the mouse button is one that WoW recognizes it you can set it up to pass that binding.

If for whatever reason the mouse button it is not recognized by WoW if you have a Logitech mouse you can use SetPoint to change the Mouse button to a keystroke.