The installer requires you to create a password during product setup, therefore there's no default password. If you have the administrator account password, you can access the user administration features at /admin (at the end of your fisheye URL). If you haven't reached out to see if there are other Fisheye / Crucible administrators are your organization (in the IT department for example), now would be a good time to do so.
If it turns out that no-one at your organization has the admin password, you can follow these instructions to reset it. I would just caution to double-check that you're modifying the QA environment instead of the production environment.