You may this issue if Visual Studio was installed on your machine prior to SQL Server 2008 R2.

To manually add Management Studio, visit: http://www.microsoft.com/en-us/download/details.aspx?id=7593

If the link has changed, search download Management Studio for Express 2008 r2 on your favorite search engine.

Once downloaded, run and follow the instructions for new installation. Under the features option you will see Management Studio as an option. Check the box and continue the installation.