At the bottom of the train loc is a black baseplate that has a battery cover with a button you need to push to open it and then you can take out a battery holder for 6 AA batteries. Replace all 6 batteries and look where the + of the batteries should go.
This instructions show the button and the location of the batteries:
https://www.lego.com/cdn/product-assets/product.bi.core.pdf/4539725.pdf
Is this it? I tried to find one which best matched your description - If you have the model number, the LEGO website offers a great service to find old manuals
LEGO Imperial Flagship 10210 Manual PDF
Try the LEGO website, they offer their customers great support for all their current and old products, if your cant find it on their site i'm sure they can help you by sending in an email Rinstruct The home of instructions