Accrinet programmed a custom application to import MLS listings into a custom Freedom module. www.ggar.com and www.realestateinberks.com/ are using this application. Both sites were designed and built from scratch. Unfortunately, due to varied client demands and differences between RETS servers, the application must be heavily modified for each site.
The core of the application consists of the open source PHRETS library available at github. This tutorial should help you get started using PHRETS. www.retsmd.com provides a way to discover the structure of the RETS server from which you will retrieve MLS data.
Information you will need to obtain from your client includes login credentials to their RETS server, the exact resource, class, and field names of the data they wish to display on the website, and any filter conditions specifying which listings to display.