There are numerous ways of doing this. Choose the one that is most convenient.
Make the person the Focus Person by double-clicking on their box. Within the "Spouses & Children" tab you can click on links in boxes near them - such as "Add Child", "Add Wife/Partner" etc - to add children, spouses or parents. If the person already has one or more spouses and one or more sets of parents, click on the "Add More Relatives..." link at the bottom of the window and follow the menu options.
To add a new spouse, choose an empty spouse tab and click on the "Add Spouse/Partner" link. To add children for the person by a given spouse, choose the correct spouse tab and click on the "Add Child" link in the table of children at the bottom of the window (you may need to resize the Property Box to able to view this table properly). To add a parent, click on the 'Up' button in the Property Box toolbar.
See How to Add Relatives by Click-and-Drag in Diagrams.
Select the person you want to add a relative to, click on the menu, and then choose , or as appropriate.