You can install plugin in one of two ways:
Automated Installation
Step 1: Navigate to Dashboard >> Plugin >> Add New
Step 2: Search for BuddyPress
Step 3: Install and Activate the plugin.
Manual Installation
Step 1: Download BuddyPress from here.
Step 2: Via FTP upload the plugin inside the wp-content/plugins folder.
Step 3: Navigate to Dashboard >> plugin >> installed Plugins. Activate plugin.