I had this problem too and the reason is I didn't set the reserve fuel, look for it as well

Check your target altitude on the CLB page

It amazes me how you guys find these old threads...thank you jbcallender

I'm a newbie to PMDG, so I'm learning all of these little tricks that you guys discovered a couple of years ago....love the NGX by the way.

Press the "clb" button on the FMC, fill the info and if already filled, "exec" light should be lit. Press it and then VNAV should arm

So I am reading this years later and I had the same problem--turns out there were no V speeds shown either and although I had entered everything correctly into the FMC I had not pushed Exec button. As soon as I did that V speeds were shown and VNAV would arm! Bryan

Moved my flight controls around and the issue was resolved. I had my headphones on my yoke.