To add a Plugin you must click into Settings and then click on Plugins.
In this way you enter a page where you can select the plugins you wish to use.
You can add a plugin simply by clicking on + next to the name.
By clicking on + the plugin is selected and goes from the left side of the page to the right.
Then once is on the right side of the screen click on the name of the plugin to enter its configuration page.
All the available plugins are currently free. In the near future some of them (for sure Alloggiati, Istat, Sef, Ine, Invoices and Broom Paper) will require a minimal price. The Board and the Team plugin will be always free.