There is no automatic way to switch the users' uSpace on when the site is initially set up. This is so that the students can be introduced to it when the school is ready to do so (some schools utilise it from the outset, whilst others introduce it as the year progresses).
You can activate uSpace on a group by group basis. This can is carried out by going to Administration > Group Admin and searching for and selecting the group you wish to activate. In the subsequent View Group window, activate uSpace by clicking the 'Enable' button under uSpace Access.
uSpace can also be enabled (and disabled) from the View Class List window, located from the Page Tools icon in ClassSpace pages.