installing phpbb3 inside wordpress blog

I’m new to wordpress and love its features.which is why I’ve installed it in my website to make it my home page.Now i want to add a phpbb3 forum in my website which can be accessed by clicking on the menubar.my question is-

1)in which directory do I install phpbb3(since wordpress is installed in the root).

2)how can I add access to phpbb3 forum when clicked on the wordpress meny item(named forum)?

I’ve heard a lot about WP-united but I’m still not clear as to what it is exactly.It integrates phpbb3 with wordpress but does that mean I have to install phpbb3 first and then wordpress and then wp-united to integrate both?In my situation where my blog is set up first,can wp-united help me run a phpbb3 inside it?if yes,then how?