default will only be set for NEWLY created work items..

if you run synchronization on your work items it should update any that pre-exist.Remember you can set a default in two ways. You can use the default literal and unassigned literal values in the enumeration. or you can use the attribute Customization option. I didn't see what version you are running.select the default values.
add -- in the provider drop down you can select enumeration list and then select the enumeration field and it will display your options. Just remember if you set a field with a default value you want to ensure that you have not flagged it as mandatory(required). If you have then set an enumeration value as default to something like (please select) not a real value. Hope this helps