Spreasheet and Sheets must be created in Google Sheets prior to import.
To import an existing sheet, first sign in, select the spreadsheet (search icon at right of the spreadsheet name) and select ‘Import a sheet’ from the Spreadsheet menu.
You will then be asked for the structure of your data. If no primary key is defined in your data, a new column will be inserted in the first row.
Advanced configuration (such as file synchronisation, relationships, ..) is editable by the ‘Spreadsheet Settings’ menu, after the sheets has been imported.
Through this menu, you can also delete the sheet (and its view) from X-Sheets and reimport it.