You'll first need to connect your Twitter and Pixorial accounts. You can do this by going to your email address at the top of the screen, then the Social Networks page, and selecting "Twitter" from the dropdown under "Add Account." Select the "Connect with Twitter" button. You'll be asked to sign in and accept permissions which will allow Pixorial to post videos to your Twitter feed.
Now that the accounts are connected, return to your Pixorial library and select the video you want to share. Select the "Action" dropdown and then select "Twitter". A popup will appear allowing you to add a comment or change the title. Then you just hit Post and your video is off to Twitter!