You can claim your free credit easily through the ISIC website.
To do this:
1. Visit the ISIC website
2. Find and select the Hostelworld €5 Benefit
3. Click the link to get to the Hostelworld website
4. On the Hostelworld website, sign up for a Hostelworld account using your ISIC card details and your credit will be applied to your account When you receive your credit, you’ll be able to start using it straight away.