If you believe that you have encountered a bug in WordPress MU or in any of its plugins, then it is a good idea to report your findings to the developer so that they can fix the problem for you and any other users of the script or plugin.
For WordPress MU, you can find support at http://mu.wordpress.org. You can also read in-depth information about WordPress MU at http://codex.wordpress.org.
Most plugin developers will provide their contact details (either a web site address or an email address) in the plugin's readme
file. So, if you have an issue with a specific plugin, check that file for details. Some developers maintain a presence on the WordPress MU, BuddyPress, and bbPress forums, while others prefer support requests and bug reports be sent via email or as a comment on their blogs. There are some premium plugins on wpmudev.com and, if you have an issue with those, you should use the support features offered on that web site.
Before you raise a bug report, it is a...