If you have installed the latest version of WP ShareThis Plugin then -
Go to your WordPress admin page:
http://YOUR WP BLOG/wp-admin/options-general.php?page=sharethis.php and then
On the 3rd Step -> There are options given to add your Twitter Handle and Username
Just enter your handle name and username and save the changes.
And when you click on the Twitter button next time, you will see your Twitter handle.
Here is the screenshot of the WP ShareThis Settings Page -