If you're running Oracle Web Cache on your Portal server, the default port where you will find it seems to be 9400. So if your url is localhost, the Web cache should be at localhost:9400.
The front page at that port is usually just an info page, in the right-hand column you need to click on the link called "OracleAS Web Cache Manager" to actually launch the Web Cache UI.
Based on my experience so far, the main action you'll be doing is "Basic Content Invalidation". That's where you can clear everything out of the cache if you need to force re-generation of all the objects.