OK… I'm at a loss with your statement.
It's clearly shows how and where it's suppose to go…. in that slot where the arrow points to it. I don't understand how you can't see it.
I don't think it's the instruction guide is terrible as you say it is. I honestly believe you're having a hard time how to read the instruction guide and comprehend how & where parts suppose to go. Everything is right in front of you. Look at the picture, look at the cab you've assembled. It's like reading a blueprint. It's not rocket science.
Looking at scalemates, I found a similar instruction guide. I'm guessing you're on step 16.
Look at the cab you assembled. Do you see Part C31? Good. It goes on the LEFT SIDE FRONT !!!
You should see a small slot hole there. Do you see it????? Glue it there as shown on the instruction guide!!