If the opponent is not on already your system you can either...
1. Add a team in the "manage other teams" area of the site (accessed via the main fixtures and results page) so they appear in the "opponent" drop down when you come to create a fixture
2. Or if you are already creating a fixture and you realise you have not added a team you can click on "Add other team" button on the right handside of the screen to enter the team
Both options have the same effect of adding an opponent to your website opponent drop down menu.