Hi Ben
That usually happens when you are in compatibility mode. If you open an old excel file, a .xls, excel 2010 enters compatibility mode and makes these new features unavailable (assuring thus that when you save the file it can still be opened in the version of excel it was created with).
If this is the problem, you can convert the old file to a new format simply be saving it as a .xlsx (if it has no vba) or as, for ex., a .xlsm if it has vba and then you'll be able to use the sparklines.