Activation will happen automatically only if your Administrator has enabled auto-activation (Admin > Event Settings > Event Details). If auto-activation is not enabled, then a staff member must activate the bidder using the "Actions on Bidders" menu at the bottom of the Patrons > Bidders > Manage page.
One activation credit is needed to activate one bidder. Once activated, the bidder can place as many eBids and purchase as many fixed price items in the eCatalog as he likes. eBidder activation credits are included with the plan you purchased.