Adding Custom buttons, in ASP

I have a list box with several values listed aligned next to my FCKeditor. I would like to beable to double click a value in my list box and have the value inserted into the FCKeditor window at the current location of the cursor.
I have been working on this for several days and have gotten no where.

Here is a pic of what the page looks like.
http://www.webselfstorage.com/lettertemplates.gif

Thanks for your help

Wooanaz