Using a Multi select box as dependancey criteria

Hi, I'm trying to use a multi select box as criteria for another fields visibility. When I attempt to create the formula in Studio the field/variable for the multi select box isn't listed, in fact Sugars Formula builder returns an Unknown field error if I enter it's name manually. I have created a dependency formula manually in the Extension/Ext/..../Vardefs/fieldname.php file and am testing now to see if it actually works. Does anyone know why the formula builder doesn't recognise the multi select type field?

Thanks for your time and patience..