how? I imgmount it and nothing happens.

imgmount a x:\path\name.img -t floppy

ok, but now what? it won't open.

Maybe you could read this:
http://www.abandonia.com/vbullet/showthread.php?t=14406

That's not how you describe a problem with DOSBox.
Exact syntax of commands you entered.
Exact text of error messages.

My bad here, didn't tested it .... :o ... thought it was the usual img.

But it's a PCbooter game so:

boot x:\path\donkeyko.img

This will boot dosbox into the game directly.

Should you still have problems, read Fift's post before asking further.

Ok, thanks! that's good for the future, because Donkey Kong isn't that good on PC XD It worked, but it was extremely fast.

Lower the cycles to about 500.

I only downloaded this to see the 50m level that was cut out of the NES port, just to see it goes straight to 100m, but referred to as 50m. Also the jumping is really weird and so are the walking physics. Press right to walk right, press it again to stop? Stupid scheme.
I consider it a porting disaster.

The review is also very bad, in my opinion. It is not professionally written at all. The beginning is full of complaints about the characters name, and doesn't put the research in whether it is intended to be Mario or not [Protip: It is]. A crucial part of a review is research.