BuddyPress is a “network only” plugin. That means that you can only “network activate” it. But, the way BP works, you generally only show BP content on a single one of your blogs – we call that the “root blog”. By default, it is blog number 1, the main blog in your network. For this to work, just activate a BP-compatible theme (like BuddyPress Default).
If you want your community/BP content to be on a blog other than the main blog, you’ll have to define an alternative root blog. In wp-config.php:
define( 'BP_ROOT_BLOG', 6 );
where you replace ’6′ with the id of your desired root blog.