Amazon RDS Postgresql adding new extensions
You can't add your own extensions to RDS, at least not ones that require superuser rights (like anything with C code). This is one of the downsides you accept in exchange for convenient management.

If the extension only includes simple plpgsql and sql functions you can add the functions manually. That is not possible with anything requiring superuser privileges, including any extension with C code.

jsonbx adds functions and operators. The source contains C code, so there is no way you can load this on RDS. Ever. At all. RDS doesn't allow you to run anything that might give you superuser access or access to the underlying VM that PostgreSQL runs on, nor does it provide a way to upload extensions even if it would permit you to load them.

You will need to switch to running a standalone PostgreSQL server (say, on EC2), persuade Amazon support to make the extension available as part of their supported set of extensions, or make do without.