If you have put your content on a network drive, you need additional setup steps. Configuring an account that has access to the network drives in the configurator usually works to make streaming work. However, if you have also put your databases on a network drive or streaming still doesn't work, you need to run the service as a different user account.
Start the Services configuration program
Scroll down, search for "MPExtended Service" (not "MPExtended WebMediaPortal"!) and click properties in the context menu.
Go to the "Log On" tab. Choose the "This account" option there, and enter the details of your own user account.
Click "Apply". You might get a warning that your user account has been granted the log on as a service permission, this is OK.
Choose "OK" in all popup windows like the one below.
Now, restart the service. Right click on the "MPExtended Service" again, and choose the Restart option.
If everything went correct, you should see your own user account in the "Log on as" column.