There are plenty of resources to help answer support queries. Following are the main ones:
Forums - The forums are one of the most popular destinations for getting MantisBT support. Start off by searching the forums for your questions, if not found, then go ahead and submit a question.
Mailing lists - Available mailing lists are "mantisbt-announce" for announcements, "mantisbt-dev" for development issues, mantisbt-lang for localization () and "mantisbt-help" for general help/support questions. There are public archives for such mailing lists. Note that only members of the mailing lists can post to them, hence, subscribe to the lists before you attempt to email them.
IRC - The IRC channel is mainly used by developers to engage in in-person discussion. The recommended tool for IRC is XChat (for Linux), XChat 2 (for Windows). However, you can also use Mibbit to connect to IRC via your web browser. This is also useful when your work firewall blocks the IRC port (although there are other workarounds involving tunneling to fix this issue). Many people prefer to use IRC to ask questions to the developers and other users who are in the IRC channel.
Wiki - The MantisBT Wiki has information related to "How To (recipes)", FAQ, feature requirements, etc.
Search - A good way for locating an answer for your question or finding more information about a topic is to search accross all MantisBT website and the Internet via Google or Live.
It is important to note that support questions should not be sent directly to MantisBT developers or through the MantisBT contact us pages. Use of "Contact Us" page or emailing the developer directly is available if you are after a paid support or consulting service.