How do I setup custom ad locations? I'd like display further down - rather than after first post/first topic.

More like the fifth topic down etc

Did a small search. Seems like you can add a code like 

					{{if $postCount == 5 AND $advertisement = \IPS\core\Advertisement::loadByLocation( 'ad_topic_view' )}}
						{<put here your ad id>}
					{{endif}}

within forums->forum->topic template. You will see a very similar code inside it. I think you can put just after that.