You can add opportunity in two different ways.
By creating new Clients,
1- Add a new client. (see: How to add New Clients?)
2- Fill in the Opportunity area in the Add New Client screen.
3- Opportunity will be added to your client when you click on the Check 
button in the upper left corner.
By editing client information,
1- Open the INFO tab of the client. (see: How to edit the Client Information?)
2- Fill in the Opportunity area in the Edit Client screen.
3- Opportunity will be added to your client when you click on the Check button in the upper left corner.