Log into your account. Locate the "Page Elements" part of your blog administration page. This is usually located in "Templates" or a similar area. Check the box reading "Show Ads Between Posts." Configure the settings to make the adverts suit your blog. Choose from pre-set sizes and choose how often adverts appear. Click on "Save" to apply your settings.
He's using phpbb... As for your original question, here's a really good article for you to read: www(dot)phpbb(dot)com/kb/article/advertisements-in-phpbb3/
But he wants to place ads between posts in forum, not in blog. Anyways thanks Darthmaul for helping us.