Because our system uses one big multisite network, individual clients cannot install their own plugins and themes. However, we have pre-loaded all installations with a variety of plugins and themes you can activate. If you want to install a plugin that isn't available, you can request it on our plugin request page. However, we will only install plugins that are secure and reputable.
If you want to install a theme that isn't already available (such as a custom design that you purchased/made), please contact customer support and we will install it for you.