When you install SQL with Basic Management Tools, the installer does not allow you to go back and install the Complete Management Tools. The option will be grayed out.

In order to install the Complete Management Tools, insert the installation media for SQL, then open a command prompt and run the following...

setup.exe /FEATURES=Tools /Q /INDICATEPROGRESS /ACTION=Install /INSTANCENAME=SQLSERVER2008R2Ent /BROWSERSVCSTARTUPTYPE=Automatic /AGTSVCACCOUNT=”NT AUTHORITY\NETWORK SERVICE” /IACCEPTSQLSERVERLICENSETERMS

Note: Replace "SQLSERVER2008R2Ent" with the name of your instance.
Note 2: The above command is for SQL 2008R2. SQL 2008 does not support the /IACCEPTSQLSERVERLICENSETERMS switch