Make sure the object has UVs and a Material assigned to it.
You also need to have a texture projected onto a model in order to use the UV Unwrap tool. This tool is useful for unwrapping complex geometry that does not work well with projection mapping. Once the UVs are unwrapped, you can use UV Edit, UV Transform, and UV Fuse to piece the UV back together.
Using UV Unwrap
For specific parameter help see the UV Unwrap node help.