Log out, go to your events page, go to the event you have organized, click on Register.

See also:

http://help.wildapricot.com/display/DOC/Extra+event+registration+costs 

http://help.wildapricot.com/display/DOC/How+to+add+a+new+event

http://help.wildapricot.com/display/DOC/Customize+event+registration+form