Download SSMSE 2008 here.
The Management Studio is now built into the main SQL Server 2008 Express installation routine.
To add SSMSE, ensure that “Perform a new installation of SQL Server 2008” is selected.
Select “Management Tools – Basic” from the “Feature Selection” dialog box when prompted.
Note: if there are any issues running the installation routine, try applying SP2 and then re-try.