1024 x 600 is not a standard resolution. It sounds like you are using a raw LVDS LCD panel that does not have an EDID. If your panel had an EDID, then you would have 1024 x 600 support automatically.
You need to create a DTD using the CED utility. You will need to discover the parameters for your panel and set it up. YOu might get it to wor using the "simple parameters" option but it is better to use the configuration the panel manufacturere recommends.
Hope this helps.