Hi there,
By the way, I can't remember if there's a way to workaround this. Will ask the AppStore specialist!
For now, you indeed have no workaround to browse an external device into this CrossDJ version... as sylMV rightly sayed, this is due to latests Apple's restrictions on MacAppStore distributed applications.
But,A workaround seems possible, currently working on it, the feature should be back in a future update.