define('DIR_WS_ADMIN', '/mynewname/'); //and as according to Jan's instructions. This step seemed a bit weird as the original syntax for the first define line reads : "define('DIR_WS_ADMIN', 'catalog/mynewname/');"
I think the problem is that you left out 'catalog'.
Normally, installations of oscommerce go into a 'catalog' folder, which is annoying because there's no good reason for it, and if you don't know how to point your domain properly, your site's URL will be www.mysite.com/catalog/ -- so most of us get rid of the 'catalog' part during the installation by putting everything that *was* inside catalog up 1 folder.
Therefore, yours probably needs the 'catalog' part, and you will need to use this logic when following the instructions.
Unfortunately this is just one of those things that weren't told to you and you had to learn from experience