I want to know how to add a stylish “latest post” slider to my WordPress blog using themes like Genesis or Thesis.
I'd start with this plugin:
If it doesn't work, just keep looking. You'll find one you like. As MacWitty said, there are many to choose from.
Easies it to use a plug in. There are lots of them and the cheapest has less features but maybe you don't need so many or need the one in those package. As I do not know what you want to do, except display latest post, I can only recommend you to check out the wordpress.org plug in page