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
I had this same error, and pg_config was in my path
pg_config
You can try
pgxnclient install --pg_config "c:\path\to\pg_config.exe" $extensionName
You almost definitely need to run cmd.exe as an admin
cmd.exe