Adding Partitions to cube In SSAS via VS 2013
I've found a solution!!! :)

Thanks to Darren Gosbell(MCC, MVP) and bayFX BITALENT(Partner) on MSDN.

This appears to be caused by a missing dll. For VS2012 / SQL 2012 running the following command fixed this for me.

copy "c:\Program Files (x86)\Microsoft SQL Server\110\Tools\Binn\ManagementStudio\SqlWorkbench.Interfaces.dll"
"c:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies"

For Visual Studio 2013 ( Version 12.0.21005.1 REL ) and SQL Server 2014 you can use this paths.

Copy "C:\Program Files (x86)\Microsoft SQL Server\120\Tools\Binn\ManagementStudio\SqlWorkbench.Interfaces.dll" file to "C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\PrivateAssemblies" folder.

Restart Visual Studio and Voila!