SteemPress transmits your WordPress post via an html POST command, which appears to require curl. This may require some combination of having your host install curl and configuring WordPress to allow it. Not exactly a walk in the park.
It is possible that this won't be a requirement in the future.
cURL is installed on my server and I still get the same error.
Try out V1, available now from the WP plugin directory. See the announcement here.