Where you input the topic in the back-end, make sure you select the Forum that it should appear in on the top right of the editing page.
I just had to work this out for myself too…
I think you can use this wherever you need to in your topics loop or template / whatever..
<? $curTopicStatus = bbp_get_topic_status( $topic_id );
if($curTopicStatus == 'publish'){ ?>
This topic is still open
<? } else { ?>
This topic is now closed
<? } ?>
If you learn to copy function names and do a search for them through your whole bbpress plugin folder, you can pretty much find out how anything works, this is how I worked this out. Think I had to go through a trail of 5 or 6 files until I worked it out