To make sidebar appear on a desired category and contain a desired attribute, you must follow next 3 simple steps:

1) Make the desired category “anchor”: Go to the admin panel -> Catalog -> Manage Categories.

2) Add filterable attributes: Go  to the admin panel ->Catalog -> Attributes -> Manage Attributes.  Add attribute which you wish to filter by (Detailed explanation about adding/managing  attributes -here). Make sure that you choose “Filterable (without results)” or” Filterabe (with results)” in”Use In Layered Navigation” field (To do so, you must choose “Dropdown” or “Multiple Select” or “Price”  in “Catalog Input Type for Store Owner” field).  Also don’t forget to add some options (on Manage Label/Options tab). Finaly, make sure that you added those attributes to some attribute set (Detailed explanation about adding/managing  attribute sets – in next articles), remember the name of this set for next section.

3)Apply attributes on some products: if in the previous section, you set “Filterable (without results)” for the attribute, you will not see this attribute on the sidebar, until you will have some products that has a value for this attribute. So, create new product (choose tha attribute set that you chose in a previous section), or update existing product (that has this attribute on it’s attribute set), so that the field of the attribute(s) you added, will contain some value. Don’t forget to set a desired category as a category of this product.

Now, on your desired category page, you should see a sidebar, that contains the attribute you added.

(Note: those 3 steps are 100% effective for clean Magento installation. If some modifications where done to the code/design files, those steps may not suffice.)