You can find the newest code / instructions on the silverstripe-ecommerce google project - http://code.google.com/p/silverstripe-ecommerce/wiki/InstallationAndConfiguration

Could you be more specific about ecommerce? if you are talking about the ecommerce module then you need SilverStripe installed before the module. If you want to use SilverStripe with your existing ecommerce system (perhaps not the silverstripe module) that is a bit tricker.

I already have Silverstripe installed, and I then tried to install Ecommerce. I followed the directions that you sent to me as a link, and for some reason Ecommerce changed my website. The home page is all white with text indicating fatal error. I then got rid of ecommerce from my site directory, and now my website homepage is fine, but I saved a copy of Ecommerce in hopes of installing it correctly. I guess I do need help installing ecommerce. The link you sent helps, but for some reason the directions didn't work for me.