I installed Python 2.7.8 and pgxn client. And I tried to run this statement from command line from the bin folder and path is setup correctly
pgxnclient inst
Bakground: pg_config is the configuration utility provided by PostgreSQL. This utility is used by various applications.
Solution:
Set the path. System Properties > Advanced
PATH:C:\Program Files (x86)\PostgreSQL\9.4\bin\;
From this point on, any application that requires pg_config can find it here.
I suggest this method because the build is directly from the author. You can also find viable alternatives for standalone pg_config builds.