from the menu>edit>preferences>media. there are options for metadata. i think it should work either way, but could change it to try. do the other types (integer, boolean, and real) all appear with grey boxes too, or can you add values to those? does this happen with new projects? i take it that the default metadata fields work properly?
if you create a new schema does it work, or does it effect all new schema's?
do properties work when you add one to the default metadata, the top one, Premiere Pro Project Metadata?
if its just a problem with a schema previously created, you can close premiere and delete it from here
C:\Users\<name>\AppData\Roaming\Adobe\XMP\Custom File Info Panels\4.0\custom\