Plugins Bringing Site Down

Everytime I try to install a WPMU plugin it brings my entire site down.... Anyone else have this problem?

Here is the error message I get:

Warning: Cannot modify header information - headers already sent by (output started at /home/comereco/public_html/test/wp-content/plugins/buddypress-ads/bp-custom.php:23) in /home/comereco/public_html/test/wp-includes/pluggable.php on line 865