Here are the steps.
1) Make sure that the field ustock is in your product file and lineitem files. You can discover this by searching for an item in inventory maintenance and editing a sku. The ustock field will be at the bottom of the page. If not there, you need to ask support to update your database.
2) Go to storesetup.wws and click the inventory tab. In the 'activate oversell' feature field enter a value such as: WHD:MGM:WPI - This will tell the system that the WHD catalog for brands MGM and WPI can be 'oversold'. These brands are the same brands you will have uploaded to your product file or manually added one at a time.
3) Import a product file using wwsitemsupload.wws
That should do it!