My grandmother is turning 100 years old, how do I get the President to send her a letter regarding this mile-stone?

Here's what I found...

To receive a greeting from President Obama, send your request to the White House Greetings Office, Room 39, Eisenhower Executive Office Building, Washington, DC 20502. Fax 202-456-2461. Or go tohttp://usgovinfo.about.com/blgreetings.htm
You can arrange to have President Barack Obama send signed greetings and congratulations to US citizens for many of life's milestones. The White House will send signed greeting cards and recognition to US citizens to commemorate special events and accomplishments. Requests need to be received six weeks prior to the event, in most cases. Greetings will be mailed from the White House about ten days before the event.
All Greeting requests must be submitted in writing (no emails) to:
The White House
Attn: Greetings Office
Washington, DC 20502-0039 
Include the full name(s), address and title (Miss, Mrs., Ms., Mr.) of the recipient, plus the date and event being celebrated. Also, include your full name and phone number in case there are any questions.
The White House Greetings Office will honor citizens with special presidential greetings and acknowledgments for 80th and subsequent birthdays, 50th and subsequent anniversaries, the birth of a child, Eagle Scout or Girl Scout Gold awards, weddings, bar mitzvahs and bat mitzvahs, and it will send sympathy cards for a death.
Requests should be sent at least six weeks before the event.
The hand-addressed greeting will be mailed one week before the event.

Good luck,
~Jada~