Clarify that question please. If you want to run Buddypress you’ll need your own server – shared hosting at a pinch – as it’s a little more than a simple plugin nothing else that I’m aware of comes anywhere near the functionality that BP adds. What access if any does wordpress.com give to plugins.
It’s possible that someday WordPress.com will provide their users with an option to install BuddyPress on their blogs (for an additional fee, I imagine). But until that happens, your only choice is as @hnla has stated–running your own copy of BP on a self-hosted WP install.
@grow-stories this is an 8 month old thread you have dragged back up. please, in future, if you have an issue you require help with you must start a new post/thread and describe the problem in detail along with any applicable info such as version numbers , custom themes, plugins in use.
You activate the plugin in the usual place for plugins, you visit the BP menu section and configure any options you want such as forum install, extended profiles etc, then visit the theme section and activate the bp-default theme or suitable BP compatible theme you might have.