Importing e density onto structure

Hello,

I have been using CASTEP to run energy and Cij calculations on some alloys using a 2x2x2 supercell, I would like to superimpose the electrondensity onto the structure  but the button in the analysis|electron density| dialogue box is not active even though the .castep output file is selected in the results file dropdown menu and I can save the results to file.

Thanks

Andrew