Have Read the Manual, Getting an activity to RUN Options

[Blue Train]

I must congratulate all the people involved in getting a program like AM running.
I lost interest in MTST about 2.5 yrs ago. This was due to the fact that I had some 200 odd activity-files downloaded which I could not use. Being the fact that one needs to have extra rolling-stock.
Last night I came upon AM and really thinks it is the best ever since a slice of bread!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I then went and got hold of an activity and loaded it into MTST.
Then ran AM and cleared all the "UNKNOWN" stock.
Now for the challenge to see if Am really works.
I followed the procedures carefully and got to the screen which decided to replace with my own selection.
I then found stock with a "+" in front and new that they were the problem one's.
To the left of these +-files was onother "plus" sign.
I then clicked on this and saw some "replacement-stock" that is available.(hope I am still on the right track)
This is the problem:
Then I also saw some "+-files" with no plus-sign to the left of it.
So in my opion are these rolling-stock not replaceable and if so then you cannot run the activity??
Or did I miss a procedure somewhere down the line?
I shall appreciate some advice.
Thank you from South Africa