If you're using a mutator or custom classes, you'll want to add the system  package to your Server Packages. To do so, open up NerfEd (it is located in your Nerf System directory). Open your Preferences (under the Options toolbar) and expand the option Advanced. Then expand Game Engine Settings then Server Packages. You will see the default things that load on a server. DO NOT MODIFY those packs, rather, hit add and add the package you plan on using. For example, if you wanted to use the mutator Raj Arena you need to type RajMutators into your newly created slot.

Note that you can delete the server package if you are not using it anymore. Remember, NEVER leave a blank Server Packages slot. Always delete any empty slots or slots containing mutator packages you do not wish to use anymore. If you can't get a Server Package slot to delete, open up your Nerf System directory, open Nerf.ini, then find the line that reads ServerPackages = and delete the one that is blank or is no longer needed. NEVER delete or modify the 5 default Server Packages.

These are the 5 default packages that should be there at ALL times:

NerfKids
WarBots
HuntBots
RaceBots
Vox