Astrometry.NET has no configurable settings. In order to use it, just ensure that it is a checked option in the Chain Solver Settings. The only requirement is a reliable Internet connection.

Note: While you can place Astrometry.NET anywhere you'd like in the Chain Solver's priority list, it is highly recommended that you place it at the end (and therefore use it only as a last resort). This solver is, by far, the slowest of them all.

Note: Using this solver means that you are subject to the status of a remote server's availability (and the availability of all the servers between you and it). In order to sever this dependency, it is recommended that you install and use a local (running on your machine) version of Astrometry.NET called ANSVR.