The first thing you probably want to do is log in to your admin area. This is the part of your auction site where you can manage your site and its users. You will find the login details in the email you received with the subject 'Easy.Auction :: Your Easy.Auction installation details'. Log in to your admin area at yourdomainname.com/ea-admin, have a look around and play with the settings. To change the site's categories go to Settings > Categories.
To create auctions log in at the general user part of your website at yourdomainname.com/user/login.
Single or Multiple Seller mode
Once logged in at yourdomainname.com/ea-admin you can go to 'Settings > General Options' to the left side of the admin area. There you will find a setting called 'Grant right to sale to new users by default'. If this checkbox is checked all new users will be able to auction items on your auction site. If this checkbox is unchecked new users will not be able to auction items. Other users can only place bids!
Single Seller mode
If you choose to be the only seller on your site by keeping 'Grant right to sale to new users by default' unchecked then please allow at least one user to auction items (probably yourself). In the User menu you will find an option called 'Manage user rights'. You can grant a user the right to sell by switching the button 'Sell rights Y/N' to 'Yes'. You probably want to call this user 'Seller' or '(Your company name)'.
After you have granted one user the right to sell you can login with this user at yourdomainname.com/user/login and start auctioning! Other users that who register from now on will only be able to place bids: they cannot sell anything on your auction site and won't see the 'Publish an ad' link like you do.