You can add money to your c-card at the Housing and Dining Cashier’s Office or online [1]. You can also add money to your c-card with cash using the c-card machine on the 2nd floor of the Library, across from the elevator. You can also make c-card deposits at the Depot, Library Café, South Campus Marketplace, BSS Marketplace, and the Giant's Cupboard.
Links:
[1] http://c-card.humboldt.edu/