How do I get GSX to read a new add on scenery file?
You don't do anything, it's done automatically.
Its not reading the right bgl file there for it want work since its saying I'm not parked in the right position.
The problem is likely your scenery.cfg has an error, usually because you edited it by hand, FSX is more tolerant towards error, but for the file to work with GSX, it must be clean. Check for things such illegal comments or similar in your scenery.cfg
Enabling logging in the "Troubleshooting" section of the "GSX page" and having a look at the Couatl.LOG file, might help finding what the problem might be.
