Well, the getting started guide is the most detailed documentation there is for showing the thumbnails.
there are 3 key ingredients:
1) In the cart object, make sure you have a column for thumbnails
2) In the add to cart button, make sure the thumbnail column is being given the thumbnail file name
3) In the cart display manager select the thumbnail column and select the option to display as an image. you will also need to supply the image directory.
If this is not working for you, It would be more constructive if you could provide a copy of your cart page, the add to cart page and a link where I can see the problem to troubleshoot things.
