Error installing sql server 2008 management studio express

Hello,

I need to install sql server 2008 management studio express on a sql server 2k8 R2 instance.

First of all, I've downloaded this one from this link: http://www.microsoft.com/en-us/download/details.aspx?id=7593 (for 32 bits).

Once I've the downloaded file I run this file and choose the first option (New sql server stand-alone...) in the installation part. Finally, in the installation type I choose the option 'Add features to an existing instance of SQL Server 2008' with the instance selected. I push Next and then I get the following error: 'The instance ID is required but it is missing'.

Can anybody help me?

Thanks a lot.

Regards.