When you upload BuddyPress the default theme is automatically added to your available theme list in Appearance > Themes.
However, when using WordPress Multisite you would normally only make the BuddyPress Default theme available for your main site as you only run BuddyPress on one site.
Making your BuddyPress Default theme available for your main site only
If you want to have the BuddyPress default theme available for only your main site running BuddyPress and no other site on your network you need to:
1. Go to Themes in the network admin dashboard
2. Select No next to BuddyPress Default and then click Apply Changes
3. Now go to Sites > All sites in the network admin dashboard.
4. Locate your main site and bring up action menu by hovering your mouse over the domain name.
5. Click on the action link Edit below the domain name.
6. Select the BuddyPress Default theme and click Update Options.
Activating your BuddyPress Default theme on your main site
BuddyPress themes are generally only used on the site that is running BuddyPress plugin (this is normally the main site on a WordPress Multisite network).
1. Login to your dashboard
2. Go to Appearance > Themes
3. Click on Activate below BuddyPress Default to activate the theme
4. Your theme should now be activated and you should see it when you visit your main site.