Thomson/Reuters, the providers of EndNote Web, require that all users of campus subscriptions log on from "authorized" computers every 12 months. An "authorized" computer is one on campus or one accessing Web of Science through the database login. Since alumni do not have off campus database access, the only alumni access to EndNote Web via an authorized computer is on campus.
If you cannot come to campus, you may wish to investigate a personal subscription to Refworks, a personal copy of the EndNote software, or the services on the Alternatives to EndNote Web tab.