When you've made a selection of data from the Data Feeds service then you download it in .CSV format.
You can use many different applications to open a .CSV including text editors such as Notepad, Sublime or TextEdit; as well as Microsoft Excel. (Bear in mind, though, that you have to open the file as .CSV in order to prevent Excel from dropping leading '0' characters on some numeric fields.)
Here is a sample of the output from one simple query:
Postcode,Stat Name,Stat Value
AL100AB,Age__20_to_29,20%
AL100AB,Age__30_to_44,19%
AL100AB,Age__45_to_59,13%
AL100AB,Age__60_to_64,6%
AL100AB,Age__65_to_74,9%
AL100AB,Age__75_to_84,6%
AL100AB,Age__85_to_89,2%
AL100AB,Age__90+,1%
AL100AB,Household_Number_of_Cars_or_Vans__2_Cars_or_Vans,14%
AL100AB,Household_Number_of_Cars_or_Vans__3_Cars_or_Vans,4%
AL100AB,Household_Number_of_Cars_or_Vans__4+_Cars_or_Vans,0%
AL100AB,Gender__Male,52%
AL100AB,Gender__Female,48%
AL100AD,Age__20_to_29,13%
AL100AD,Age__30_to_44,25%
AL100AD,Age__45_to_59,13%
AL100AD,Age__60_to_64,6%
AL100AD,Age__65_to_74,0%
AL100AD,Age__75_to_84,0%
AL100AD,Age__85_to_89,0%
AL100AD,Age__90+,0%
AL100AD,Household_Number_of_Cars_or_Vans__2_Cars_or_Vans,25%
AL100AD,Household_Number_of_Cars_or_Vans__3_Cars_or_Vans,0%
AL100AD,Household_Number_of_Cars_or_Vans__4+_Cars_or_Vans,0%
AL100AD,Gender__Male,56%
AL100AD,Gender__Female,44%