When you create an attribute, you are able to decide whether you would like to include it on the Advanced Search page and/or in keyword searches.
If you tick the box 'show on search' then it will appear as a separate search field option on the Advanced Search page. 'Include in keyword searches' means that the values given to your new attribute can be searched via the main 'keywords' field in the Advanced Search page.
If you would like to change these options for existing attributes, click on the 'edit' link next to the desired attribute. It is possible to control the visibility of your attributes on the Advanced Search page by accessing the "Display Attributes" tab from the Admin - Attributes area.