You get the Ring by killing Gollum - he runs around the map, but is invisible, just like in the unmoddet game.
Then you have to send the Ring "into" the Ring-Hero (to learn who they are: look at the description at the fortress), like you had to the fortress before.
Tu build Durin, you have to put the Ring into the Mausoleum, which can be build by your Builders.
But a general tip: Read the descriptions, if you understand them - they're very helpful, use a translator, if you need.