What do I do with the compost!

[CakeyCakeyCakey]

I got the keys to my new house today (rented) and in the garden which is all paved but has a raised bed to one side and big planter at the top end (previous owners filled half the garden with big planters and grew veg)
There's a big box with what I assume is compost in, i lifted the lid and it all looked rotten I'm not much of a gardener but I feel as though I should use it to keep up the garden that's there, and te big empty planter might get some potatoes planted or something.
would it be ok to use it on the garden? Is it supposed to stink? And what should I put in it to keep it going?
I'm intending to just use the bits I'll need and maybe give mil some instead of her buyin it for her garden.

I'm a complete novice lol

Well, our compost bin:
Doesn't smell of anything other than soil.
Is topped up with uncooked fruit and veg waste from the garden (plus some of the teabags and egg shells), along with some of the garden clippings and lawn mowings - I know this isn't applicable!
I have recently emptied most of it, and dug it all into the flower bed. We did stop topping in up mid summer tho, so the whole lot was composted. Otherwise you need to desperate the composted material from the "in progress" material. Some people have 2 composts going - one composting, one taking the kitchen waste.

Would it be ok to use do you think? Do I just chuck it ontop of what's there or dig it in?

Dfil was the gardener in the family he'd have come and sorted it and told me what to do.

Dig it in or use it as a mulch on top, either is good. But beware - I moved to a new house recently and used the well rotted compost I found in the compost bays there. Up sprang thousands of weeds! They had clearly been chucking all garden waste in them including weeds. I wished I'd used it on a small trial area first instead of a thirty foot border.