Adds a "Tweet this" link to posts, shortens URLs with tr.im or TinyURL and sends the user to Twitter.com.
Yes, PHP on the server you install the plugin on must have enabled the setting allow_url_fopen. If not, the plugin cannot shorten the URL.
If you aren't logged in when you click the "tweet" link, it's not a problem. First, you'll be redirected to the login page at Twitter.com. Once you've logged in, you are redirected to your home page at Twitter.com and the URL will be ready for you in the status field.
Unfortunately no. This will be available in a later version.
If you get this error it means that PHP on the server you use isn't allowed to access other web pages using file_get_contents(). Unfortunately, this is the way FishyTweet connects to the selected URL shortener to shorten the URL to your post. The plugin will still work but URLs won't be shortened.