Try this : http://www.iberia.com/us/

When I check the timetable I only see 3 flights / week for IAD-MAD (IB6168). And it looks as if the return goes to Boston first before flying to IAD.

You can get to this website by clicking on the “Iberia in / en” in the left top corner of main site.

For the FF program, click on the “Iberia Plus” link on the right hand side. But in my experience Iberia Plus is worth nothing unless you fly business / first (a lot).

Good luck!