For Developing Applications using Silverlight 5 Beta you should have Visual Studio 2010

Steps to install Silverlight 5 Beta
1) Install Microsoft Visual Studio 2010 Service Pack 1

http://go.microsoft.com/fwlink/?LinkId=210710

2) Install Microsoft Silverlight 5 Beta Tools for Visual Studio 2010 Service Pack 1

http://tinyurl.com/Silverlight5Tools

Get Documentation for Silverlight 5 Beta
Microsoft Silverlight 5 Beta Offline Documentation Package contains the MSDN developer documentation for Silverlight 5

http://tinyurl.com/Silverlight5BetaDocumentation


http://preview.tinyurl.com/Silverlight5BetaDocumentation

For More details visit
1. www.silverlight.net
2.
http://www.silverlight.net/getstarted/silverlight-5-beta/