Like with any software program it installs to a path on your hard drive that being the C Drive normally or Local Disk C found under My Computer (Pre XP and Windows XP) or Computer in Vista, 7, 8, 8.1 and Windows 10
if you are using a 32-bit of Windows it normally installs to Program Files or if you are using a 64-bit of Windows then it normally installs to Program Files (x86)
In this case MSTS installs to the default path of Program Files Microsoft Games Train Simulator or Program Files (x86) 64-bit versions of Windows as stated above and in my previous post
Or if you have changed this path during installation of MSTS into your custom path then that's where MSTS will be installed to
meaning that the path you have chosen to install MSTS to is the path you need to add in Open Rails
Unless your willing to move Routes, Trains (Trainset+Consist), Sounds into the main Open Rails folder located within either Program Files (32-bit system) or Program Files (x86) (64-bit system) but this is just too much playing around and you have installed MSTS onto your computer so the easiest way to fix your problem from what I have guessed is adding MSTS by locating it via it's installation path to Open Rails as explained above in my previous post
It would be best that you do explore your computer and get to know things about it such as it's folders and files
there's heaps of information all over the internet that will help you learn your computer or you could buy an expensive book at your local bookshop hahaha

either way it's all helpful information as I don't want to write down everything relating to your computer lol

Or you can simply click the Start button and go into Computer from the right side menu
if however you don't see Computer there right click and select Properties and then click Customize and normally Computer should be displayed as LINK and once you've done that you can access "Computer" folder and files in Windows 7
Anyway, like I said I'm not going to explain everything about your computer I think it be best that you do learn about it more to find your way around things and it will help you located MSTS and Open Rails files and folders with some luck