Navigate to the Services tab in the left side bar, select the service category you would like to add a new service too.

Note if you would like to make a new category see the Add new service category how to guide.

Select the New service button.

Fill out the New service form,

and select the Save button.

You can now see your new service under the service category you selected.
Tip: If you've got a lot of services to add, you can import them from an excel file.