BuddyPress.org

@Chouf1. Are you talking about your custom.css or bp-custom.php (as your last comment indicates)?

Put custom.css in your _inc/css/ directory. But, more importantly, un-comment the custom.css file (/* @import url( _inc/css/custom.css ); */) in the style.css file.

Put bp-custom.php in wp-content/plugins/, if you have one.