To get the discount, as Drat (above) said, you just show them the card at boarding. However, it’s pretty common for places like Greyhound to not check the discount despite them saying that they will check at concession.
Specifically for the ISIC discount…(From http://maphappy.org/2014/12/the-hard-real-truth-about-isic-card-benefits/)
I bought five separate STA tickets over the course of 2010 to 2012 without an ISIC card. I only got asked for the card maybe once, from Air Canada, I believe it was. My response? I pretended I forgot it at home and asked if I could show my student card instead. The check-in agent waved me through.
Hopefully this helped! 🙂