https://wordpress.org/extend/plugins/wp-multi-network/ is probably no longer compatible with the latest buddypress.
Did you try the patched version: http://alantait.net/2011/07/27/wp-multi-network-plugin-fix-for-wordpress/ ? In the comments of that page @johnjamesjacoby mentioned ( 12/02/2012 ) that he planned to update wp-multi-network soon…..
@boonebgorges recently wrote http://wordpress.stackexchange.com/a/45682/7401 which seems particularly relevant to your question. In the post he mentions Networks + as an alternative way to enable multiple networks but it is a premium plugin.
