• Download bp-private-message-rate-limiter-x.x.zp (x.x are version numbre)
• Unzip it
• Upload bp-private-message-rate-limiter to wp-content/plugin directory
• Visit Dashboard->Plugin (or Network Admin->Plugins if you are using BuddyPress network wide)
• After Activating, Visit BuddyPress->Settings(On WordPress it is under settings tab, on WordPress Multisite, It is inside NetworkAdmin->Settings tab)
• Set the time and the allowed count and that's it.