You have used code $bbuserinfo which is the Current User Browsing the forum, which is you as the ID is 1. To use the code in the first post only and to Guest and Registered user then use this instead. is_member_of($bbuseruserinfo, 12)
Code:
<if condition="$post[postcount] == 1 AND is_member_of($bbuserinfo, 0,2)">
<div style="padding:0px 0px $stylevar[cellpadding]px 0px">
Adsense Code Here</if>
I guess it would work. Not checked though.
Shishir