Re: No input file specified.
First things first, do you have WPMU 2.71 running correctly before you try and install BuddyPress?
Secondly, try manually installing BuddyPress by downloading the .ZIP file from the WordPress plugins repository (https://wordpress.org/extend/plugins/buddypress/).
Do not trust the automatic installer! Put the entire contents of the folder in /wp-content/plugins/ and then login to the WPMU admin area’s plugins page and activate.
Let us know what happens.