When you pop up the dialog for a new nose cone, even before picking from the database via the upper right dropdown, there is a default list of materials available. These are hardcoded in Databases.java. However if you click Custom and add a new material to the list, it gets persisted using Prefs and lands in (on OSX)
~/Library/Preferences/com.apple.java.util.pref.plist (binary file)
At this point it doesn't look like there's any way to add materials to this dropdown list via a .orc file - a severe limitation. There is code in Databases.java that adds database listeners for line/surface/bulk materials, but the actual listener in MaterialStorage.java only adds the material to the preferences, and only if the material is flagged as user-defined.
There is also no UI to remove a material defined in the prefs, though there's code in the MaterialStorage DatabaseListener that will remove the pref on elementRemoved() firing. But you can use Apple's property list editor included in the Apple Developer Tools X Code package from developer.apple.com/xcode, or with TextWrangler from barebones.com.
See also (on OSX)
~/Library/Preferences/openrocket.favoritepresets.*.plist
which look like they will contain the presets you designate (by checking items in the database) for the various component types (nose cone, body tube, etc.)
After you pick an item from the database, the "Component material" dropdown still shows the same generic list...it does NOT show the list of materials defined in the manufacturer specific .orc file. This means if you change the material for a catalog component to something in the dropdown, you are not able to change it back to the original material defined in the manufacturer .orc file.
Windows We need solid information about how this works in Windows from a contributor!