I add plugins and themes also if i upload or import them to a file tell me where i can find the file and how i upload them
i can't find that file tell me how to find it
That's a directory, not a file.
if you have your forum on the server in directory "www/forum/" then in directory "forum" you have "inc" folder and in this one you can find "plugins". I'm pretty sure that you find this.
In the FTP client where you uploaded MyBB, look in the inc folder (you will see one since MyBB needs this to operate). Inside of it will be a plugins folder. If there isn't, you can create one.