The postcode for the site is PO10 7GD. As you can see in Google Maps below, this leads you to the car park at the end of Yarrow Rd, from where it is a short walk. If you tap on 'View larger map' you will be able to get directions to the car park.
The hand drawn map of Hampshire Farm Meadows shows how to get from the car park to the arboretum, along with access by foot only from Long Copse Lane at the other end of the meadows.