Hello,
The Add New button should display under the menu options as well as on the main plugin page. If you are not seeing that, you may have an issue with other plugins or the theme affecting the core of WP.
To check the theme, simply set your theme to the default one and check to see if the Add New button reappears. If so, then you will need to either change themes altogether or switch to the default them when you want to add a new plugin. If the button still does not appear, you will want to check your plugins.
To test this, turn off all plugins and see if the button appears. If so, then turn them on one by one to determine which one is affecting the WP. If the button still does not appear after turning off all the plugins, you may want to reinstall WordPress. If this is a brand new installation, simply delete the old one and then reinstall. If you have a lot of content, you will simply want to reinstall the core files on your account so you do not lose any data.
i hope this helps
