To conveniently use them, you are better off using the application-level APIs like the Java connector (JCo) and .NET Connector. Librfc32.dll is part of the SAPGUI installation, the JCo and .NET connectors can be downloaded from http://service.sap.com.
If you want to know how to use JCo or the Windows OCX provided with SAPGUI, have a look in the mySAP and NetWeaver sections of my site, http://www.logosworld.com.
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