Once you have it working, this is a great little addition to your BuddyPress install. If you get an error like the one below, there is a super easy quick fix.
Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘_bp_external_activity_refetch’ was given in...
In your (network) admin panel Plugins -> Editor, select the bp-external-activity and then bp-external-activity.php. In that doc, search for the two instances of _bp and replace with bp. The leading underscore is just a typo. How easy is that?
Here's hoping this saves some folks from headaches. Cheers to stwc for the fix in the comments of the release post.