If you are going into BWI (cheapest flights, check Airtran) there is a bus that goes from the airport to the Greenbelt metro station. You can get to the NIH from there. Don't remember the name of the bus, check
www.wmata.com for more info (this site will also help with metro information).
If you go to Dulles (second cheapest flight) you're screwed. Don't do this.
If you go to National / Reagan (most expensive flights) take the metro or the FREE NIH SHUTTLE straight to NIH. The latter takes awhile during rush hour, though. Look for details on the NIH website or PM me, and I can help you find it.