Custom nodes are containers for other nodes, and custom node definitions are saved as separate files (with extension .dyf) so that they can be shared between multiple projects and multiple users. If a custom node is unresolved in your Dynamo file, it means that Dynamo could not find its definition when the program started. Check that you have the custom node file and that it is saved where Dynamo can find it. If the custom node came from a package, make sure you have the appropriate version of that package installed.