To reproduce:
1. In the IDE, open the schema file where you want to add a complexType component. 
2. In the Schema view, right-click the root node of the schema and choose Add >
Complex Type from the pop-up menu. 
The Complex Type dialog box opens. 
3. In the Name field, type the name of the complexType. 
4. Under Type, select the Use Existing Definition radio button.
5. Select the Extension radio button, and in the list of types, make a selection. 
6. If you are extending a complexType definition, from the Inline Compositor
drop-down list next to the Extension radio button, select a compositor for the
definition. 
7. Click OK.

Notice that to complete step 6, you have to go back up in the dialog box to get
to the field, this doesn't flow very well.
To have the Extension radio button and the Inline compositor dd list on the same
level in the dialog box leads to thinking that one should be enabled when the
other is selected.