The reason for which the Electron Density is not accessible is that no electron density data is available. Data in the .cube files can be anything e.g. electrostatic potential; all you have in .cube files is a 3d grid of float values, that's it. Regarding using +/- signs in the current version of Molekel, you can simply reload a second copy of the molecule and generate a new surface specifying a different isovalue. While this is definitely a workaround it's also quite handy because you can e.g. use one molecule to render all the surfaces with positive isovalues, another to render all the surfaces with negative isovalues and then toggle the visibility of each molecule to display only the surfaces with a specific isovalue.