You will access your ShowHouse account on the unique address set during the signup process. This will have been automatically generated from the agency or business name you provided, although you may have changed it.
Click the link in the welcome email or, even better, store it as a bookmark in your favourite browser since you'll be using this daily.
For example, if your agency name is My Lettings, then your login address will be something like https://mylettings.showhousesoftware.com/. Note that the https:// element of the web address indicates that the site is protected using secure digital certificates in the same way as your online banking; look out for the telltale padlock in the browser address bar.