The download from the App Store may have located itself on the Mac's Launchpad. Also look under the applications folder.
To learn the program there are the video tutorials and there is also a written manual which is listed as Railmodeller Pro Help under the help menu.
The software is decently done but like other layout software it takes a little playing with it to get used to it.
If you find something is missing from a library the creator of the app is very good at answering questions. It usually takes no more than 48 hours to hear back. Remember he is in Europe.
Hope some of this helps.
Paul