The Image Stitcher can be found in the File Menu by going to File>>Import>>Stitched Image. When you open the Image Stitcher, the previous parameter and layout settings used to stitch will be restored. If you've never stitched an image before, the default settings will be used.
The main Image Stitcher UI can be found on the right. The section on the left lists the images you have loaded to be stitched, whereas the section on the right is where you specify how the image was acquired. A preview is given of the current layout settings you have for the images you have loaded.