This section allows you to activate, deactivate, and manage the plugins that you have uploaded to your forum's inc/plugins directory.
Where is the directory?
The directory is in <your forum root>/inc/plugins
You can use your ftp program to get to it.
How did you upload the MyBB files to your server/hosting?
Go to where you uploaded the MyBB files and find the inc folder, inside that there will be a plugins folder.