Click + Add widget and select either Spreadsheets or Excel from the data sources list.

From the Select a data source panel, hover over Dropbox and click Connect.

In the pop-up window you'll need authorize the connection to Geckoboard.

Once this is done you’ll see a list of spreadsheet files you can select to build your dashboard.

If you have spreadsheets saved across multiple Dropbox accounts or users that you’d like to connect to your dashboard, click + Add account after selecting Dropbox from the Choose file panel.