What I usually do in this case is instead of doing any JOINs at all, just create two recordsets with just single table references and have one filter off the other inside the repeated region.
So the steps might be:
1) Create your category recordset
2) Create your products recordset filtered with a url parameter ($_GET["CategoryID"] for intstance)
3) Create your category repeat region (done);
4) Create your products repeat region (note: DW won't let you nest, so you can create it below and copy and paste to move the product repeated region into the category repeated region.
5) Copy and paste the products recordset code into the category repeated region as well.
6) Add this code above the products recordset inside the category repeated region:
<?php $_GET["CategoryID"] = $row_rsCategories["CategoryID"]; ?>
That should do it and will allow more flexibility than you might have trying to do it with a single repeat region and joins.