You cannot as any user in the Sandbox (or any environment) unless you ask Salesforce support to enable the "Enables Org Admins to log in as any user without having access granted by the users" permission on your Org.
You can then turn the feature on by going to Your Name > Setup > Security Controls > Login Access Policies > Administrators Can Log in as Any User. Once that is done you should have the Login action link available for all users.
Otherwise you can ask the individual users to grant you login access by going to Your Name > My Personal Information > Grant Login Access.
EDIT: As of Summer '15 the "Administrators Can Log In As Any User" feature will be available in all Orgs by default. No need to ask support to enable it!
After the Summer '15 release, administrators will no longer have to open a case with Customer Support to activate the "Administrators Can Log In As Any User" setting. Any administrator with the "Manage Users" permission will be able to control the setting directly from their org. Note: The setting will be set to "OFF" by default; administrators will still need to enable it. When this setting is enabled, administrators can log in as any user in their organization without asking end-users to grant login access.