Apart from the ribbon commands, are there any cell-level spreadsheet functions available in the Excel Add-In?

The add-in contains 2 Excel spreadsheet functions: QSERIES and QTABLE. For both functions, you will need to know the Quandl code of the data you wish to access. 
QSERIES is used with data in time-series format. To learn more about using the QSERIES formula, please see  here

QTABLE is used with data in tables format. To learn more about using the QTABLE formula, please see  here