If you are just looking for a Requirements or ReqIF-Editor, just download formalmind Studio and start being productive. Formal Mind Studio is a third-party, stand-alone build of ProR with enhancements. Alternatively, you can install ProR into any Eclipse-Installation (3.6 and later) by using the update site URL provided on the Download Page.
RMF became an Eclipse project in November 2011 (Initial Proposal). As it is still developing at a rapid pace with some parts of the API still changing, it is still in incubation. Nevetheless, the data model is stable (as it is based on the OMG ReqIF standard), and it is already used in production environments.
The Requirements Interchange Format (ReqIF) is an OMG Standard for exchaning requirements. ProR supports Version 1.0.1 and 1.1 (which are binary compatible), while code for handling RIF 1.1 and 1.2 is part of the RMF Core. ProR is used by the ProSTEP Implementor Forum for testing the various ReqIF implementations.
RMF provides a rich API to work with ReqIF models. ProR provides extension points, allowing the use of alternative renderers and behavior. Therefore, providing a seamless integration with other Eclipse tools and EMF models is comparably easy. This has been demonstrated in the FP7 research project Deploy.
A collection of useful utilities called ProR Essentials is available for free and allows, amongst other things, the correct rendering and editing of rich text (xhtml), handling of ReqIF archives (.reqifz) and more. Note that these are third-party extensions that are not part of Eclipse. The third-party formalmind Studio already comes with Essentials installed.
To get started as a user, the following resources will be useful:
The following collection of links will hopefully help you find quickly what you are looking for.