if you are modifying the newbb_viewtopic_flat.html template/
add..
<{if $topic_post.post_no == 1 }>
code for ads go here
<{/if}>
so it would look like this.. (ad after the first post... im not sure with . after the last one yet. i've been a little busy lately)
<{include file="$newbb_template_path/newbb_item.html" topic_post=$topic_post}>
<{else}>
<{*include file="db:newbb_thread.html" topic_post=$topic_post mode=$mode*}>
<{include file="$newbb_template_path/newbb_thread.html" topic_post=$topic_post mode=$mode}>
<{if $topic_post.post_no == 1 }>
code for ads go here
<{/if}>
<{/if}>