Board Thread:Questions and Answers/@comment-92.26.156.102-20130518110907/@comment-216.118.151.154-20130819012435

Make sure you have a battery with energy first. While you are holding your tool, hold shift and scroll the mouse wheel, and you should see the plasma cannon icon from the tinker table above your power tool (on your hotbar). Hold right click to charge, and release to fire.