「まだ、機内食をいただいていません」って英語でなんて言うの？

"Excuse me" is the best way to get someone's attention, especially if you do not know them. From there, you can simply state that you did not receive a meal. The flight attendant should understand and then ask you which of the options you would like.

I hope that this helps! :)

Best to put the blame where it is deserved and tell a member of crew they messed up!
The first sentence is the best.

When we don't receive our meals on the plane it can be quite distressing. Asking the crew for a meal is easy and straight forward. It is always good to remember they are there to help you. :)'

You can be polite and say - "Excuse me..." when you want a crew members attention. You can then go onto say "...I didn't receive my lunch/dinner." The crew will get one for you. Or you could also take the pressure of the crew and frame it as if it was a total accident that they missed you.

You could be feeling quite cross/angry - "Sir, I haven't received my meal. I would like the chicken now."

When the cabin crew forgets to give you your in-flight meal; then you may remind them in the following ways:
-I still have not received my meal yet.
-Excuse me; I think you might have forgotten to give me my meal

To get someones attention then the polite way is to say 'excuse me'
If you have been given something (got it) then you can also say you have 'recieved' it
To let the person know that they have forgotten you then you would say 'sorry but I think you have forgot my meal'

"I still haven't received my meal yet"
To 'Receive' something is to be given/presented something.
"I believe you had skipped my meal"
To use the term 'believe' is to think something, a more formal and polite way of phrasing this.
'Skipped' is used when something has been missed or looked over.