I installed this plugin on a new blog I setup, PPH 5.x, WP 2.9, etc.
When I enter in the API and secret key, hit save, I get a PHP error referring to check_admin_referer on line 121.
It will not save the API or secret.
For the time being I just commented out that line and the plugin worked fine.
Just wanted to give an FYI. Not sure if it is something going on with my install (it is new however) or if it's universal.