Online registration is conducted on Active.Com:

Online registration closes at 11:59 PM Eastern Time, Wednesday - April 14, 2010. Please note that Active.Com charges a separate processing fee.
Prior to March 31
15k & 5k $22.00, Scenic Walk & Kiddie K $12.00, Wee One in stroller $5.00
April 1-14
15k & 5k $26.00, Scenic Walk & Kiddie K $15.00, Wee One in stroller $5.00
April 15 through Race Day
After 11:59 PM Eastern Time, April 14th, no online registration
15k & 5k $30.00, Scenic Walk & Kiddie K $16.00, Wee One in stroller $5.00
Note: Children in strollers on Scenic Walk must be registered with waiver per National Park Service regulations. $5.00 entry fee includes "wee-ones" t-shirts for these little folks.