You must associate a node tree diagram to an IDEF0 model.
1 Open the IDEF0 Model definition that contains the Function/Activity that you want to decompose. Normally, you decompose the Function/Activity that is drawn on the model's Context IDEF0 diagram, but this does not have to be the case. The IDEF0 Model definition can be opened from the browser (in the All Methods tab, expand the Definitions group and search for IDEF0 Model) or from within an IDEF0 diagram (open the IDEF0 diagram and select Edit, Diagram Properties, then click the Definition button by the Model (IDEF0 Model) field).
2 Within the IDEF0 Model definition, click the Choices button of the Node Tree Diagram (Node Tree) field. The currently existing Node Tree diagrams in your encyclopedia are displayed in the dialog, including the one you just saved.
3 Select and drag the Node Tree diagram into the Node Tree diagram (Node Tree) field. Release your mouse button to drop the name into the field. Click OK to close the dialog.