From the Namecrave knowledge base:
1. Contact your Web Hosting Company and find out from them the Name Servers you need to use
2. Login to your Control Panel. For Customers -
http://manage.namecrave.com/customer
For Sub-Resellers -
http://manage.namecrave.com/reseller
3. Search for the Domain Name whose Name Servers you want to modify, by searching for the Domain Name at Products -> Domain Registration -> Search.
4. Click on the Domain Name for which you want to Modify Name Servers.
5. Click on the Modify Name Server button. Feed in the Name Servers you want (upto 13 name servers) and click "Submit"
If the Domain Name is Locked, you will have to Unlock this Domain Name first by referring to Lock / Unlock
If at this step you get an error stating Name Server is not yet registered, this means that the Name Server you are using has not been registered as a Name Server with the Registry. Please contact your Web Hosting Company and ask them to provide Name Servers that are valid and registered with the Registry.