see
Installing Plugins guidance
basically
plugin files are to be added to inc/plugins folder of MyBB files server.
if plugin consists of single
php file then that is the
simple method. but plugin might
consist of several files. in that case we have to
follow folder structure of plugin pack.
if plugin files are located in a folder named
Upload then
its contents should be transferred
by following folder structure. that
Upload folder itself should not be transferred.
after adding the plugin files you have to visit admin panel >> configuration >> plugins
page to install & activate newly added plugin. then proceed to configuration >> settings
page to find the settings related to newly added plugin and setup the plugin settings.
youtube has some videos to help (you can search for
MyBB plugins installation)
ALSO
you can
first install Plugin Uploader and then try using it for installing the other plugins