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The updated posterous requires the user to enter and store their api_token and site_id, in addition to usename (which is the e-mail address associated with your posterous acct) and password in their settings->connector settings->posterous.
To find your api_token, go to http://posterous.com/api and in the top of the page, under "Working with Users" / "Retrieve the current user's information", click "view token". A pop-up will show you your api_token.
Be aware, however, the first page will return only 1 site_id, so if you have more than one posterous site, this is not going to be helpful. For those with only a single, personal posterous, if works.
See http://posterous.com/api for instructions to retreive a list of sites and their relevant data (including site_id), or, if you use gnu/linux or any other system with bash, use the script at the bottom of this, the second page linked above.
If you have multiple blogs and can't use a bash script, you'll simply have to refer to the posterous api page linked above.