You access existing standalone projects/files from the Connection page (accessed by clicking "Select connection" from the login page).
The Connection page looks like this;
The screen above is shown if the civil pro data file option is selected - this will allow you to select an existing file by clicking on the "Select file" button, navigating to the directory your file is in and selecting the file.
Click the "Connect" button and you will be returned to the log in screen.