masterbranch requires Mantis 2.0.x, while the
master-1.2.xbranch works for Mantis 1.2.x.
Extract this repo to your Mantis folder/plugins/Slack.
On the Slack side, add a new "Incoming Webhooks" integration and note the URL that Slack generates for you.
On the MantisBT side, access the plugin's configuration page and fill in your Slack webhook URL.
You can map your MantisBT projects to Slack channels by setting the plugin_Slack_channels option in Mantis. Follow the instructions on the plugin's configuration page to get there. Make sure the plugin_Slack_channels configuration option is set to "All Users", with type "complex". Example value for this setting:
array ( 'My First Mantis Project' => '#general', 'My Second Mantis Project' => '#second-project' )
You can specify which bug fields appear in the Slack notifications. Edit the plugin_Slack_columns configuration option for this purpose. Follow the instructions on the plugin configuration page.