When entering stable details in the trip itinerary screen, there is a drop down list for Stable.

To add a new stable for this drop down list, go to People/Add person

Select Stable from the roles drop down list. Click next to "Roles" to close the drop down list then select "Next"

Enter the name of the stable in the "Company" field and other contact details such as phone number, and address if required.

Scroll to bottom of the page and select "Save". The new stable will appear in the trip itinerary Stable drop down list next time it is opened.