Load your pics into DSS, click check all then click to register them making sure that stack after registering is not selected.
Once the pics are registered click on one of the light frames at the bottom to view the image, on the right hand side you will see some icons click on the comet one.
Then using the mouse wheel zoom in to the pic and move the cursor over the centre of the core, left click to select, next click on the disk icon on the right to save it. You need to do this on all your light frames.
Now click stack checked pictures on the left of screen and within this click stacking parameters. Select comet and choose which option you want then click ok.
It should now stack aligned on the comet.