Palette Selector is available only if the selected image has a color palette.
To verify if image has a color palette Open raster manager-> Utilities\Color table and see if you get the message raster image has no color palette.
To Create a raster with a color palette export your current raster to a format supporting palette color mode (e.g.: 256 colors).
1. Open Raster manager: File\Save file
2. Select options
3. Raster save as Setting\Format tab select Color mode 256 colors and ok
Once this is done selector palette is available as your image as a color table added