can u compete in the prelims at nats if u did not get a seed

Yes. Prelims are open to anybody.

Before you could but in 2009 you have to qualify in a regional contest. So in order to compete in the Prelims or Finals you have to qualify.
Quote From The National yo-yo Contest WebSite
Qualification Information:

To compete in a Championship Division (1A, 2A, 3A, 4A, 5A), a competitor must qualify by competing in and placing in a National Yo-Yo League Regional Contest in the 2009 Contest Season.

Oh ok. Thanks. Didn’t know that rule change.