Nikon D40 mirror lockup problem?

I can't get my mirror to lock in the up position. I've done it before so I know it worked in the past. I can go to the setup menu, then to mirror lockup and I right arrow the control button to get it to check the box, but when I hit ok, instead of getting the screen display that has the printed directions, it unchecks the box. Hit ok again and it checks the box. When I hit the shutter button, the screen goes back to the original display screen, in other words, back to square one. The battery showed full charge, but I charged it fully and that makes no difference.

Anyone know what I'm doing wrong or if it's something in the camera (I suspect it's me)? I'm planning on taking off on a motorcycle vacation Wednesday and I need to clean the screen. Thanks!