After upgrading Poster (App Store version) I cannot export from Aperture. I get screen with: "Post with account: New Account" - the only choice available. I tried to reinstall exporters - did not help. I can send pictures directly from Poster to flickr.
Sorry you are having problems. One thing that could cause this is if you still have the demo version of the app from this site still on your Mac.
Use Spotlight to search for Poster… if you find more than one version of the app, move the oldest one to the Trash. If not, let me know and we can dig a bit deeper.