I presume its works the same as CS2. So try this, open all images into Elements that you want to merge together, then use the move tool (in CS2 it is situated at the top right corner of the tools palate) next whilst holding down the shift key move one image at a time onto another (using the shift key will align the images up 100%). One you have done this, you will notice that each image is on a different layer, so that you can adjust each image by erasing the bits that you don't want to use from that particular image allowing the bits from the image underneath to be seen. Finally when you have got the image that you wanted go to Layers and flatten the image.
Disclaimer I'm no expert in Photoshop so maybe others will tell you to do it differently or easier 