Hi, when you use the importer, once you have selected your own csv file to upload, and clicked the ‘upload file and import’ button , you will see a table appear for you to match the columns of your file to the appropriate columns of wp-ecommerce.
Below that table are options to check/uncheck if you are merging products, importing images etc.
Instructions can be found in the ‘read me’ file included in your product importer download and also on this page..
There are example csv’s to download to if you need them.
I hope that helps a little.