When the wordpress landing page plugin is activated, you cannot install new plugins.
You get the following error:
An unexpected error occurred. Something may be wrong with WordPress.org or this server’s configuration. If you continue to have problems, please try the support forums.
I have tested this with a fresh install of wordpress and with no plugins active except for the landing page plugin.
To install a new plugin in the interface I need to deactivate the landing page plugin before I can.