Can I download percentage changes instead of raw data, using the Excel Add-In?

This is only possible for data feeds that are in time-series format. You can do this and other calculations by going through the Quandl Formula Builder (accessed via the Get Data button) and selecting ”row-on-row % change” under the Filters tab. Other options include “row-on-row change”, “cumulative sum” and “normalize starting at 100”.