We've a guide on connecting to your Timpani site here.
Once you've connected successfully, extract the theme or plugin files locally -- if they were delivered (for example, from a paid-plugin service) as a zip file, you'll need to unzip them. Once you've done that, in filezilla browse on the right-hand panel to either the 'plugins' or 'themes' folder, as relevant. On the left hand panel, browse to where you deployed the plugin files.
Each plugin and theme should be in it's own folder, within the relevant 'theme' or 'plugin' directory on our servers.
If the theme or plugin extracted as a single folder, just drag-and-drop it from the left hand panel, to the right.
If it extracted as all the files and folders within the theme or plugin, right click in the right-hand-panel, and select "Add folder". Name the folder to match the plugin, without any spaces. Double click the new folder to enter it on the right hand panel. Then you can drag-and-drop the files from the left hand panel, to the right.
Once that's done, you just need to login to your WordPress dashboard, and activate the relevant plugin or theme.