You have to make sure that you catalog has a ship in there if it is your own or if you downloaded it.
To get to the catalog press \"i\" for your inventory then on the bottom of the window you should see a \"Catalog\" tab. Click the tab then you should see something that says \"Create a new entry\". You have to be in a ship inorder for that button to work. Once in a ship click the button. Save the ship under whatever name you would like. Then go under \"Catalog\" and click on admin. Once under admin click the ship that you just saved. Then click permissions and then make sure the enemy box is check. Then the enemy will use the ship you just saved (well spawn as that ship). So if you want easy enemies build simple ships, if you want hard enemies build compliacated ships.