MyBB allows the installation of plugins, which extends the basic functionality of MyBB. To find a plugin, you browse MyBB’s official site for plugins in their official Mods and Plugins section.
To install the plugin into your directory, follow the instructions that appear in the “Information” box in the plugin’s download site. If there are none, just upload the contents of the folder to the /opt/bitnami/mybb/inc/, and if there are just PHP files upload them to inc/plugins/
Finally, to install the plugin to your database and activate it, in the left sidebar in “Home” or “Configuration”, look for “Plugins”, and then look for “Install and/or Activate” below the “Controls” columns (inside the “Inactive Plugins” category):