Is it when the runner navigates past any ICE and begins his access, or is it at the end of access?

Case in point, if using the basic criminal runner and accessing HQ for the first time, do you have the two credits from his ability at your disposal to use for trashing a card?

A run is declared successful after the Runner has passed the last piece of ICE (if any) and decided NOT to jack out. That means you will have the two credits for trashing. It also means cards like Medium gain a token before the access step happens.

Awesome. Thanks!