Help Topics is a PDF of the documentation for PSSE.
Click the Application Program Interface
This is the "PSSE API" documentation. You'll find a huge list of the available functions in here.
Search for FNSL
We want to use the FNSL command, so search for FNSL inside the document and click the link.
FNSL description
We're interested in Python, so you can read the "Python syntax".
If at the top of the file you've typed:
import psspy
then to use fnsl you'll need to type
psspy.fnsl()
not simply
fnsl()
Like it is shown in the documentation! I personally prefer to type psspy.fnsl - because then it is obvious which library the 'fnsl' function came from.