I access the metadata editor in 9.3.1 this way -
In ArcCatalog, find the shapefile/featureclass using the left-hand window in Catalog.
Select the file, and in the right-hand Catalog window, click the Metadata tab.
There is a metadata editor toolbar with icons for editing, importing, exporting metadata. The toolbar can be turned on by going to the View menu and selecting Toolbars and putting a check mark next to Metadata.
The toolbar also has a pulldown menu of style sheets, including FGDC.
You can change the default metadata editor in Catalog by going to the Tools menu, then Options. At the bottom of the Metadata tab is a pulldown menu.
There is an excellent editor for 9.3.1 called "Three Tab Editor 1.9" available free from the Coeur d'Alene Tribe, native Americans, online.