Couple of things to try out:
- Check that the UDID you added to the portal is ticked for use under the AdHoc provision. When adding a new UDID it's possible to forget to manually update the provision profile with the new UDID.
- Don't create the link yourself and don't use a plist. Just drag the ipa you created to a site like http://www.diawi.com/ and share that link with your tester.
Good Luck