Where can I see the projects I've dismissed from https://itch.io/library/things-to-rate? I want to set them back to normal so I can see everything again.

I do not believe there is such a function. You can choose how long they will be dismissed - so if you have chosen a time period, they should reappear after that time frame. If you chose until next interaction or forever, then that is what you have chosen.

If you closed those pages recently, you may be able to re-open them via your browser history. On their project page, you can then rate them. (Or after your next interaction with them, if you have chosen that before)