The parts of your brain involved in symbolic language are less active when you're asleep, and since interfaces like that are basically all symbols your brain has a hard time understanding any symbols it remembers or making sense of anything it can put together. The part that remembers stuff is still going strong though.
So basically you know what a computer is and how it should work, but you're trying to use an interface you've never used before in a language you don't speak that was designed by a person with uncertain notions about where icons and windows should go or how they move.

In most cases it's easier for the dream to just say "and then you clicked the button to do the thing", like it does with signs and stuff.

My dreams managed to conjure my Mac login screen which, pretty impressive. Let me try to log in for quite a while as my panic increased.