Two different formulas you can use:
Excel 2003 or older:
=SUMPRODUCT(--(A2:A25="April"),--(B2:B25="John"),C2:C25)
Excel 2007 or newer:
=SUMIFS(C2:C25,A2:A25,"April",B2:B25,"John")
If you have Excel 2007, you can also use entire column references in the SUMIFS (and SUMIFS is significantly faster than SUMPRODUCT):
=SUMIFS(C:C,A:A,"April",B:B,"John")