If that is a mortgage interest for your primary or secondary home - you should report it on the schedule A line 10 or 11 (depending if you were provided with reporting form 1088 or not) - http://www.irs.gov/pub/irs-pdf/f1040sab.pdf
Please let me know if that is a mortgage interest on investment, business or rental property - in that case there will be different reporting.
you should not look at form 1040A.
You need to look at the schedule A that I provided for you above - here is the link again - http://www.irs.gov/pub/irs-pdf/f1040sab.pdf
Moreover you will not be able to use form 1040A if you want to deduct the mortgage interest - you should use form 1040 instead.