Adsense after first post in showthread

I've been wrestling with putting Google adsense after my first post in showthread with special conditions:

I want it to show to guests only
I do NOT want it to show in forum 6

Right now, here is what I have:

<if+condition="($post[postcount]+%+20+==+1)+AND+($bbuserinfo[userid]+==+0) AND ($forum[forumid] != 6)">
<if condition="THIS_SCRIPT == 'showthread' ">
Google code
</if></if>

This doesn't work. Everyone sees the ad when I put this code at the bottom of my postbit template.

[Kathy]

Sorry, I have no idea where the + came from when I copy/pasted the code into the post.

[Snake]

Thanks!