Okay, so as I understand it you can't find the 158 in 'free roam' mode. Well, the reason for that is that the free roam scenarios came with the route, so in general only default stock appears by, er, default.
I'll go through this step-by-step, and I apologise now if the instructions below seem patronising or you've done them already!
Step one is to check that you have the upgrades Mk1 and Mk2 installed. You can get these by going to
www.railsimulator.com and registering - you'll then find these on the front page of the site.
Step two: do you have the 'dev tools' installed? Again these are available on the RS website and although it sounds as though the tools are only for geeks and content creators, in practice they are needed to do quite basic editing such as changing scenarios (or adding a 158!).
Step three: open your desired 'free roam' and at the bottom of the screen, on the 'fly up' window, click on the globe at the right end:
Hey presto! You're in the 'World Editor'.
Now, take your mouse to the top left of the screen and another 'fly out' will appear. See that little train icon, top right of the fly-out? Click on that.
Woah, that message looks a bit scary! Don't worry, just click on 'yes'.
Now we've unlocked all the fly-outs to put more stock on the track, create new services, all that. First, take cursor to centre-left and it's another fly-out:
This the REALLY important bit. Can you see at left a little blue cube with an orange traiangle on it? Click on it, and at top right you get yet another fly-out. Go through the list, and make sure you find and select Ben Laws (who created the 158 ) as an add-on supplier. I'm doing this from memory, so I'm not sure but I think his supplier moniker is 'SD9L' or something similar. Only now will the 158 appear in the list of available stock.
Okay, now start to look at the stock just to the right of the big blue cube. It's split into engines, stock and consists so first let's click on engines and that brings up a list of the power available. Click on a loco (this is the green Deltic downloaded from Rail Simulator's website) and magically a train appears that you drag to the track where you want it to sit:
Click on it, let it go, and it's there.
Okay, so we have now placed a train on the track. But so far it's static - you can't drive it. How do you make a train driveable? Er, add a driver... Back to that top-left fly-out, this time click on the face.
Now click on the loco and hey presto, you have a driver icon above the cab.
You don't have to do the next stage, but so that you have a proper name for your service when you press F6, just double-click on that icon above the loco and yet another fly-out appears:
Type a service name, and press enter.
Okay. now go down to the bottom right corner of the screen to the triangle that looks like a VCR 'play' button:
Yes please, if we say 'no' we lose ALL our changes and we're back to square one.
The thing reloads...
...and there it is, we now have two steamers and one diesel to choose from.
That's it - away you go!
As I said above, I hope that's clear without teaching granny to suck eggs